CVE-2016-2110: auth/ntlmssp: maintain conf_flags and required_flags variables

We now give an error when required flags are missing.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=11644

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