CVE-2016-2110: auth/gensec: fix the client side of a spnego downgrade

New servers response with SPNEGO_REQUEST_MIC instead of
SPNEGO_ACCEPT_INCOMPLETE to a downgrade.

With just KRB5 and NTLMSSP this doesn't happen, but we
want to be prepared for the future.

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>
