lib: Introduce winbind_xid_to_sid

lib: Introduce winbind_xid_to_sid

This does not merge a winbind communication error into
"global_sid_NULL" (S-1-0-0), which by the way non-intuitively does not
go along with is_null_sid(). Instead, this just touches the output sid
when winbind returned success. This success might well be a negative
mapping indicated by S-0-0, which *is* is_null_sid()...

Signed-off-by: Volker Lendecke <vl@samba.org>
Reviewed-by: Christof Schmitt <cs@samba.org>
Bug: https://bugzilla.samba.org/show_bug.cgi?id=13813