auth3: fallback to "sam_ignoredomain" in make_auth3_context_for_ntlm()

auth3: fallback to "sam_ignoredomain" in make_auth3_context_for_ntlm()

This is in the spirit of the "map untrusted to domain" parameter: We
fall back to the local SAM when we get a non-authoritative NO_SUCH_USER
from our domain controller. With this change we can implement
"map untrusted to domain = auto".

We should not strictly need 'sam' before 'winbind', but it makes
it clearer to read and has the same effect.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=2976
BUG: https://bugzilla.samba.org/show_bug.cgi?id=8630

Signed-off-by: Volker Lendecke <vl@samba.org>
Reviewed-by: Stefan Metzmacher <metze@samba.org>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>
Autobuild-User(master): Andrew Bartlett <abartlet@samba.org>
Autobuild-Date(master): Mon Apr 10 05:04:03 CEST 2017 on sn-devel-144