Replace most USE_IPV6 with run-time support probing

Replace most USE_IPV6 with run-time support probing

This unifies the code built for IPv4-only, dual-stack and split-stack.

 * --disable-ipv6 option remains, however it now prevents the run-time probe

 * Probing previously done in ./configure at build time is now merged and
performed run-time on every startup. IPv6 is enabled or disabled based on
the underlying OS support for sockets and setsockopt operations required.

 * Parsing and other operations which can be performed without specific IPv6
connectivity are enabled.

 * Some DNS logic alterations have had to be made to merge the split-stack
DNS and leverage it for IPv4-only mode. Otherwise the logics are unchanged
from previous dual-stack builds which have been well tested.