dsdb subnets: warn when an IPv6 address is in IPv4 embedding range

dsdb subnets: warn when an IPv6 address is in IPv4 embedding range

We fail on these ones, and it isn't immediately obvious why. Windows
also fails on *most* of them, but succeeds on "::ffff:0:0" which is a
bit strange but there you go.

Signed-off-by: Douglas Bagnall <douglas.bagnall@catalyst.net.nz>
Pair-programmed-with: Garming Sam <garming@catalyst.net.nz>
Reviewed-by: Garming Sam <garming@catalyst.net.nz>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>
Autobuild-User(master): Andrew Bartlett <abartlet@samba.org>
Autobuild-Date(master): Thu Dec 24 07:16:25 CET 2015 on sn-devel-144