fix fallback when ipv6 is disabled but resolv.conf has v6 nameserves

fix fallback when ipv6 is disabled but resolv.conf has v6 nameserves

apparently this code path was never tested, as it's not usual to have
v6 nameservers listed on a system without v6 networking support. but
it was always intended to work.

when reverting to binding a v4 address, also revert the family in the
sockaddr structure and the socklen for it. otherwise bind will just
fail due to mismatched family/sockaddr size.

fix dns resolver fallback when v6 nameservers are listed by