CVE-2016-2110: auth/gensec: fix the client side of a new_spnego exchange

Even for SMB where the server provides its mech list,
the client needs to remember its own mech list for the
mechListMIC calculation.

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>
