CVE-2016-2110: auth/ntlmssp: let ntlmssp_handle_neg_flags() return NTSTATUS

In future we can do a more fine granted negotiation
and assert specific security features.

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>
