IPV6: Add basic IPv6 support

This adds basic IPv6 support to libnfs.
Since libnfs currently only support PORTMAPPER protocol up to version 2
the IPv6 support only works if the server runs Both MOUNT and NFS protocols
on the same ports for IPv6 as for IPv4.

To get full IPv6 support we need to add support for PORTMAPPER version 3
and use it for discovery when using IPv6