r18015: Try and detect network failures immediately in

r18015: Try and detect network failures immediately in
set_dc_type_and_flags().
Fix problem when DC is down in ads_connect, where
we fall back to NetBIOS and try exactly the same
IP addresses we just put in the negative connection
cache.... We can never succeed, so don't try lookups
a second time.
Jeremy.
(This used to be commit 2d28f3e94a1a87bc9e9ed6630ef48b1ce17022e8)