CVE-2016-2114: s3:smbd: use the correct default values for "smb signing"

This means an ad_dc will now require signing by default.
This matches the default behavior of Windows dc and avoids
man in the middle attacks.

The main logic for this hides in lpcfg_server_signing_allowed().

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

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