CVE-2015-5370: s4:librpc/rpc: use auth_context_id = 1

In future we want to verify that the auth_context_id from the server
is what we expect.

As Samba (<= 4.2.3) use a hardcoded value of 1 in responses, we
need to use that.

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>
