mount.nfs: don't use IPv6 unless IPV6_SUPPORTED is set

mount.nfs: don't use IPv6 unless IPV6_SUPPORTED is set
Commit 1f3fae1fb25168aac187ff1881738c8ad53a8763 made mount.nfs start
looking up and trying to use IPv6 addresses when mount.nfs was built
against libtirpc (even when --enable-ipv6 wasn't specified).

The problem seems to be that nfs_nfs_proto_family() is basing the family
on HAVE_LIBTIRPC. I think it should be basing it on IPV6_SUPPORTED
instead.

Signed-off-by: Jeff Layton <jlayton@redhat.com>
Acked-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Steve Dickson <steved@redhat.com>