CVE-2016-2113: s4:librpc/rpc: verify the rpc_proxy certificate and hostname if configured

BUG: https://bugzilla.samba.org/show_bug.cgi?id=11752

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