Modules compatibility: removed unneeded IPV6_V6ONLY checks.

Modules compatibility: removed unneeded IPV6_V6ONLY checks.

The IPV6_V6ONLY macro is now checked only while parsing appropriate flag
and when using the macro.

The ipv6only field in listen structures is always initialized to 1,
even if not supported on a given platform.  This is expected to prevent
a module compiled without IPV6_V6ONLY from accidentally creating dual
sockets if loaded into main binary with proper IPV6_V6ONLY support.