Fixes for non-blocking with larger payload and improvements to the test and examples

[dgarske]

• Fix for continue during variable part of the header. A continue in the middle of variable would not correctly process for large messages.
• Fixed issue with QoS2 on received publish ACK getting skipped if write is already locked.
• Further improve WOLFMQTT_TEST_NONBLOCK.
• Added logic for detection of active read or write to return MQTT_CODE_CONTINUE.
• By default keep mutex locked if we tried to write. The wolfSSL TLS engine requires an SSL_Write that returns WANT_WRITE to be called with the same buffer/sz, not a different one, even if no data was sent. If user wants to enable the feature anyways they can use WOLFMQTT_ALLOW_NODATA_UNLOCK. Only the write has this logic as the issue doesn't exist for an SSL_Read.
• Fixes for using the -f [file] option with examples.
• Allow build-time override of MAX_BUFFER_SIZE in examples.
• Add packet_id to the publish log message in examples.
• Add test case for trying to send multiple publishes in the same thread.
  ZD 16769

[dgarske]

FYI: I can reproduce the error locally. Getting a disconnected due to protocol error from broker, which for some reason leaves the local broker instance running still:

Broker PID is 29060
1700688555: mosquitto version 2.0.11 starting
1700688555: Config loaded from scripts/broker_test/mosquitto.conf.
1700688555: Opening ipv4 listen socket on port 11883.
1700688555: Opening ipv6 listen socket on port 11883.
1700688555: Opening ipv4 listen socket on port 18883.
1700688555: Opening ipv6 listen socket on port 18883.
1700688555: mosquitto version 2.0.11 running
Base args: -T -C 2000 -h localhost -p 11883
1700688555: New connection from 127.0.0.1:39066 on port 11883.
1700688555: Client <unknown> disconnected due to protocol error.
MQTT Client: QoS 0, Use TLS 0
MQTT Net Init: Success (0)
MQTT Init: Success (0)
NetConnect: Host localhost, Port 11883, Timeout 5000 ms, Use TLS 0
MQTT Socket Connect: Success (0)
MQTT Connect: Proto (v3.1.1), Continue (-101)
MQTT Disconnect: Success (0)
MQTT Socket Disconnect: Success (0)
1700688555: New connection from 127.0.0.1:39072 on port 11883.
1700688555: Client <unknown> disconnected due to protocol error.

Will investigate on Friday.

[dgarske]

Tests with TLS and non-block are failing due to this bug -> wolfSSL/wolfssl#7001

[embhorn]

Update - needed to update wolfSSL first!

-----Ignore----
Seeing failures with
./configure --enable-mt --enable-nonblock CFLAGS="-DWOLFMQTT_TEST_NONBLOCK"

ASS: scripts/client.test
FAIL: scripts/multithread.test
PASS: scripts/firmware.test
PASS: scripts/awsiot.test
FAIL: scripts/nbclient.test
PASS: scripts/azureiothub.test

[dgarske]

Customer is fully happy. No outstanding issues. This PR is ready for merge.