Bring this code into line with new winbind_lookup_name() interface. I think

Bring this code into line with new winbind_lookup_name() interface.  I think
this might need a bit more work - or at least documentation.

This is certainly a worthwile little hack, as it avoids the need to invert the
group database.  I don't think we should allow unqualified domains here - as
that allows us to distinguish between (at least some) usernames and these
'special' groups.
(This used to be commit 151dd7bc6c61e19a993017e5e0b50314801e26de)