s4-dsdb: rework drs_ObjectIdentifier_to_dn() into drs_ObjectIdentifier_to_dn_and_nc_r...

s4-dsdb: rework drs_ObjectIdentifier_to_dn() into drs_ObjectIdentifier_to_dn_and_nc_root()

This make this funciton the gatekeeper between the wire format and the
internal struct ldb_dn, checking if the DN exists and which NC
it belongs to along the way, and presenting only a DB-returned
DN for internal processing.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=10635

Signed-off-by: Andrew Bartlett <abartlet@samba.org>
Reviewed-by: Stefan Metzmacher <metze@samba.org>