s3:winbindd: fix endless forest trust scan

s3:winbindd: fix endless forest trust scan

Commit 0392ebcd1d48e9f472f2148b85316a77d9cc953b effectively
disabled the enumeration of trusts in other forests.

The fixes for https://bugzilla.samba.org/show_bug.cgi?id=11691
changed the way we fill domain->domain_flags for domains
in other forests.

Commit fffefe72fcc62d9688b45f53a5327667dc0b2fe6 readded the
ability to enumerate trusts of other forests again, in order to
fix https://bugzilla.samba.org/show_bug.cgi?id=11830

Now we have the problem that multiple domains
(even outside of our forest) are considert to be
our forest root, as they have the following flags:
NETR_TRUST_FLAG_TREEROOT and NETR_TRUST_FLAG_IN_FOREST.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=12605

Signed-off-by: Stefan Metzmacher <metze@samba.org>
Reviewed-by: Ralph Boehme <slow@samba.org>
Autobuild-User(master): Ralph Böhme <slow@samba.org>
Autobuild-Date(master): Thu Mar  2 17:53:14 CET 2017 on sn-devel-144

(cherry picked from commit f9aaddcdd8f9ea648c9c5ea804f56ee3ff6c4c67)