ipv6 support

[salesiss]

Hy zerovm/UI, does udt support ipv6 or can it be implemented ?

It would be nice for ipv6 to work, having ipv6-only bindings.

[salesiss]

It's easy to edit appclient.c and appserver, for example and change a single line:
vi appserver.c
vi appclient.c

Change:
/* We only care about IPV6 results */
near line44: hints.ai_family = AF_INET6;

works fine.

But addional best practices logic for ipv6 domain name resolution, implementation of command line parameters, such -4 or -6 (like telnet) and the complicated ipv6 address syntax:
that's still is work to do, as any good linux command line tool, it demans work, and testing.

Amazing small code yours, compiled less than 20kb, but let's finish it.