CVE-2015-5370: s3:rpc_server: make sure auth_level isn't changed by alter_context or auth3

BUG: https://bugzilla.samba.org/show_bug.cgi?id=11344

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