CVE-2015-5370: s4:rpc_server: maintain dcesrv_auth->auth_{type,level,context_id}

This will simplify checks in the following commits and avoids
derefencing dcesrv_auth->auth_info which is not always arround.

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>
