trust: make sure external trust topology is correctly rendered
When external trust is established, it is by definition is
non-transitive: it is not possible to obtain Kerberos tickets to any
service outside the trusted domain.
Reflect this reality by only accepting UPN suffixes from the external
trust -- since the trusted domain is a part of another forest and UPN
suffixes are forest-wide, there could be user accounts in the trusted
domain that use forest-wide UPN suffix but it will be impossible to
reach the forest root via the externally trusted domain.
Also, an argument to netr_DsRGetForestTrustInformation() has to be
either forest root domain name or None (NULL). Otherwise we'll get
an error as explained in MS-NRPC 22.214.171.124.5.
Reviewed-By: Martin Babinsky <email@example.com>