Add IPv6 support for determining FQDN during ADS join.

Signed-off-by: David Holder <david.holder@erion.co.uk>
Reviewed-by: Jeremy Allison <jra@samba.org>
Reviewed-by: Ralph Böhme <rb@sernet.de>

Autobuild-User(master): Jeremy Allison <jra@samba.org>
Autobuild-Date(master): Tue May 12 23:35:32 CEST 2015 on sn-devel-104
