Initialize vars & change types to appease Windows/VS

[gojimmypi]

Description

Addresses these warnings encountered during a Windows / Visual Studio build:

• edit: also introduces #define WOLFSSL_MAX_8BIT 0xffU in type.h

1>C:\workspace\wolfssl\wolfssl\wolfcrypt\curve25519.h(151,1): warning C4141: 'dllexport': used more than once
1>C:\workspace\wolfssl\wolfssl\wolfcrypt\curve25519.h(151,1): warning C4141: 'dllexport': used more than once
1>C:\workspace\wolfssl\src\dtls.c(721,36): warning C4244: 'function': conversion from 'word32' to 'word16', possible loss of data
1>C:\workspace\wolfssl\src\dtls.c(732,67): warning C4244: 'function': conversion from 'word32' to 'word16', possible loss of data
1>C:\workspace\wolfssl\wolfssl\wolfcrypt\curve25519.h(151,1): warning C4141: 'dllexport': used more than once
1>C:\workspace\wolfssl\wolfssl\wolfcrypt\curve25519.h(151,1): warning C4141: 'dllexport': used more than once
1>C:\workspace\wolfssl\wolfssl\wolfcrypt\curve25519.h(151,1): warning C4141: 'dllexport': used more than once
1>C:\workspace\wolfssl\wolfssl\wolfcrypt\curve25519.h(151,1): warning C4141: 'dllexport': used more than once
1>C:\workspace\wolfssl\src\ssl_sess.c(3891,46): warning C4244: '=': conversion from 'int' to 'word16', possible loss of data
1>C:\workspace\wolfssl\src\ssl_sess.c(4167,22): warning C4244: '=': conversion from 'unsigned int' to 'byte', possible loss of data
1>C:\workspace\wolfssl\src\ssl_sess.c(4183,23): warning C4244: '=': conversion from 'unsigned int' to 'byte', possible loss of data
1>C:\workspace\wolfssl\src\ssl_asn1.c(288,36): warning C4244: 'function': conversion from 'const int' to 'byte', possible loss of data
1>C:\workspace\wolfssl\src\bio.c(1382,25): warning C4244: '=': conversion from 'int' to 'byte', possible loss of data
1>C:\workspace\wolfssl\src\x509.c(1476,59): warning C4244: '=': conversion from 'int' to 'byte', possible loss of data
1>C:\workspace\wolfssl\src\ssl_p7p12.c(951,21): warning C4245: 'initializing': conversion from 'int' to 'word32', signed/unsigned mismatch
1>C:\workspace\wolfssl\wolfssl\wolfcrypt\curve25519.h(151,1): warning C4141: 'dllexport': used more than once
1>C:\workspace\wolfssl\wolfssl\wolfcrypt\curve25519.h(151,1): warning C4141: 'dllexport': used more than once
1>C:\workspace\wolfssl\src\tls13.c(4036,17): warning C4244: 'function': conversion from 'word32' to 'word16', possible loss of data
1>C:\workspace\wolfssl\src\tls13.c(6316,20): warning C4244: '=': conversion from 'word32' to 'word16', possible loss of data
1>C:\workspace\wolfssl\src\tls13.c(10862,39): warning C4244: '=': conversion from 'word32' to 'byte', possible loss of data
1>C:\workspace\wolfssl\src\tls13.c(10866,39): warning C4244: '=': conversion from 'word32' to 'byte', possible loss of data
1>C:\workspace\wolfssl\src\tls13.c(11036,39): warning C4244: '=': conversion from 'int' to 'byte', possible loss of data
1>C:\workspace\wolfssl\src\tls13.c(11040,39): warning C4244: '=': conversion from 'int' to 'byte', possible loss of data
1>C:\workspace\wolfssl\src\tls13.c(11949,49): warning C4244: 'function': conversion from 'int' to 'word16', possible loss of data
1>C:\workspace\wolfssl\src\tls13.c(11949,57): warning C4244: 'function': conversion from 'word32' to 'word16', possible loss of data
1>C:\workspace\wolfssl\wolfssl\wolfcrypt\curve25519.h(151,1): warning C4141: 'dllexport': used more than once
1>C:\workspace\wolfssl\wolfssl\wolfcrypt\curve25519.h(151,1): warning C4141: 'dllexport': used more than once
1>C:\workspace\wolfssl\wolfcrypt\src\asn.c(13614,1): warning C4701: potentially uninitialized local variable 'finalName' used
1>C:\workspace\wolfssl\wolfcrypt\src\asn.c(13614,1): warning C4703: potentially uninitialized local pointer variable 'finalName' used
1>C:\workspace\wolfssl\src\pk.c(3666,1): warning C4701: potentially uninitialized local variable 'hashType' used
1>C:\workspace\wolfssl\src\pk.c(7961,1): warning C4701: potentially uninitialized local variable 'derSz' used
1>C:\workspace\wolfssl\src\pk.c(16559,1): warning C4701: potentially uninitialized local variable 'curveOid' used
1>C:\workspace\wolfssl\src\pk.c(16559,1): warning C4703: potentially uninitialized local pointer variable 'curveOid' used
1>C:\workspace\wolfssl\src\ssl_load.c(5045,1): warning C4701: potentially uninitialized local variable 'derSize' used
1>C:\workspace\wolfssl\src\pk.c(7901,1): warning C4701: potentially uninitialized local variable 'derSz' used
1>C:\workspace\wolfssl\src\pk.c(16607,1): warning C4701: potentially uninitialized local variable 'keySz' used
1>C:\workspace\wolfssl\wolfssl\wolfcrypt\curve25519.h(151,1): warning C4141: 'dllexport': used more than once
1>C:\workspace\wolfssl\wolfcrypt\src\dsa.c(176,5): warning C4003: not enough arguments for function-like macro invocation 'SAVE_VECTOR_REGISTERS'
1>C:\workspace\wolfssl\wolfcrypt\src\ecc.c(12596,39): warning C4334: '<<': result of 32-bit shift implicitly converted to 64 bits (was 64-bit shift intended?)
1>C:\workspace\wolfssl\wolfcrypt\src\ecc.c(12597,39): warning C4334: '<<': result of 32-bit shift implicitly converted to 64 bits (was 64-bit shift intended?)
1>C:\workspace\wolfssl\wolfcrypt\src\ecc.c(12598,39): warning C4334: '<<': result of 32-bit shift implicitly converted to 64 bits (was 64-bit shift intended?)
1>C:\workspace\wolfssl\wolfcrypt\src\ecc.c(12599,39): warning C4334: '<<': result of 32-bit shift implicitly converted to 64 bits (was 64-bit shift intended?)
1>C:\workspace\wolfssl\wolfcrypt\src\ecc.c(12600,39): warning C4334: '<<': result of 32-bit shift implicitly converted to 64 bits (was 64-bit shift intended?)
1>C:\workspace\wolfssl\wolfcrypt\src\ecc.c(12601,39): warning C4334: '<<': result of 32-bit shift implicitly converted to 64 bits (was 64-bit shift intended?)
1>C:\workspace\wolfssl\wolfcrypt\src\ecc.c(12608,74): warning C4334: '<<': result of 32-bit shift implicitly converted to 64 bits (was 64-bit shift intended?)
1>C:\workspace\wolfssl\wolfcrypt\src\ecc.c(12609,48): warning C4334: '<<': result of 32-bit shift implicitly converted to 64 bits (was 64-bit shift intended?)
1>C:\workspace\wolfssl\wolfcrypt\src\ecc.c(13590,1): warning C4701: potentially uninitialized local variable 'mp' used
1>C:\workspace\wolfssl\wolfcrypt\src\ecc.c(13275,1): warning C4701: potentially uninitialized local variable 'mp' used
1>C:\workspace\wolfssl\wolfcrypt\src\ecc.c(13413,1): warning C4701: potentially uninitialized local variable 'mp' used
1>C:\workspace\wolfssl\wolfcrypt\src\pkcs7.c(13303,1): warning C4701: potentially uninitialized local variable 'tag' used
1>C:\workspace\wolfssl\wolfcrypt\src\pkcs7.c(14151,1): warning C4701: potentially uninitialized local variable 'tag' used
1>C:\workspace\wolfssl\wolfcrypt\src\pkcs7.c(14219,1): warning C4701: potentially uninitialized local variable 'version' used
1>C:\workspace\wolfssl\wolfcrypt\src\pkcs7.c(5513,1): warning C4701: potentially uninitialized local variable 'outerContentType' used
1>C:\workspace\wolfssl\wolfcrypt\src\pkcs7.c(12110,1): warning C4701: potentially uninitialized local variable 'length' used
1>C:\workspace\wolfssl\wolfcrypt\src\pkcs7.c(12000,1): warning C4701: potentially uninitialized local variable 'contentType' used
1>C:\workspace\wolfssl\wolfcrypt\src\pkcs7.c(12013,1): warning C4701: potentially uninitialized local variable 'tag' used
1>C:\workspace\wolfssl\wolfcrypt\src\pkcs7.c(5177,1): warning C4701: potentially uninitialized local variable 'length' used

• edit: This PR does not include fixes for the warning C4141: 'dllexport': used more than once, noted above.

Fixes zd# n/a

Testing

Confirmed working with benchmark and test apps.

Checklist

• added tests
• updated/added doxygen
• updated appropriate READMEs
• Updated manual and documentation

[gojimmypi]

Jenkins retest this please

[dgarske]

Nice cleanups and extra checks. Passes my review and testing. Over to @SparkiDev to finalize.

[douzzer]

Mostly looks good! Had a few nits and a question or two.

[douzzer]

This all landed in a good place. Thanks @gojimmypi !