CVE-2016-2114: libcli/smb: let mandatory signing imply allowed signing

BUG: https://bugzilla.samba.org/show_bug.cgi?id=11687

Pair-Programmed-With: Stefan Metzmacher <metze@samba.org>

Signed-off-by: Ralph Boehme <slow@samba.org>
Signed-off-by: Stefan Metzmacher <metze@samba.org>
Reviewed-by: Günther Deschner <gd@samba.org>
