samba-tool trust: support discovery via netr_GetDcName

samba-tool trust: support discovery via netr_GetDcName

In case a remote DC does not support netr_DsRGetDCNameEx2(),
use netr_GetDcName() instead.

This should help with FreeIPA where embedded smbd runs as a domain
controller but does not implement full Active Directory compatibility.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=13538

Signed-off-by: Alexander Bokovoy <ab@samba.org>
Reviewed-by: Stefan Metzmacher <metze@samba.org>
Autobuild-User(master): Alexander Bokovoy <ab@samba.org>
Autobuild-Date(master): Tue Jul 24 09:55:23 CEST 2018 on sn-devel-144