PASSDB: add support to set and enumerate UPN suffixes associated with our forest

PASSDB: add support to set and enumerate UPN suffixes associated with our forest

Samba PDC may manage a forest containing DNS domains in addition to the primary one.
Information about them is advertised via netr_DsRGetForestTrustInformation when
trusted_domain_name is NULL, according to MS-NRPC and MS-LSAD, and
via netr_GetForestTrustInformation.

This changeset only expands PASSDB API; how suffixes are maintained is left
to specific PDB modules. Set function is added so that suffixes could be
managed through 'net' and other Samba utilities, if possible.

One possible implementation is available for ipasam module in FreeIPA:
http://git.fedorahosted.org/cgit/freeipa.git/commit/?id=cc56723151c9ebf58d891e85617319d861af14a4

Reviewed-by: Andreas Schneider <asn@samba.org>