Change our Domain controller lookup routines to more carefully seperate

Change our Domain controller lookup routines to more carefully seperate
DNS names (realms) from NetBIOS domain names.

Until now, we would experience delays as we broadcast lookups for DNS names
onto the local network segments.

Now if DNS comes back negative, we fall straight back to looking up the
short name.

Andrew Bartlett
(This used to be commit 32397c8b01f1dec7b05140d210bb32f836a80ca6)