Prefer IO::Socket::INET6 over IO::Socket::IP

While IO::Socket::IP is a core perl module (since Perl v5.19.8, or so
says corelist), IO::Socket::INET6 has been around longer, is said to
be more widely deployed, and most importantly, seems to have less bugs
hitting us.  We therefore prefer IO::Socket::INET6, and only fall back
to IO::Socket::IP if the former doesn't exist on the local system.

Reviewed-by: Viktor Dukhovni <viktor@openssl.org>