s4-repl: try harder to find the right SPN in replication server

s4-repl: try harder to find the right SPN in replication server

when doing DRS between domains, using the right SPN is essential so
the KDC can generate referrals to point us at the right DC.  We prefer
the GC/hostname/DNSDOMAIN form if possible, but if we can't find the
hostname then this changes the code that generates the target
principal name to use either the msDS-HasDomainNCs or hasMasterNCs
attributes to try to find the target DC domainname so we can use the
E3514235-4B06-11D1-AB04-00C04FC2DCD2/GUID/DNSDOMAIN SPN form.

Pair-Programmed-With: Andrew Bartlett <abartlet@samba.org>