[s4] Referrals work

[Matthias Dieter Wallnöfer]

I looked into an older bug report of mine 
(https://bugzilla.samba.org/show_bug.cgi?id=6493) and tried to implement 
those LDAP referrals. My implementation isn't fully complete yet (since 
we will need some kind of DNS bindings - see my commit comments) but I 
see it as a good start.

Also this work is located in my private repo in the "referrals" branch.

Greets,
Matthias