bus-proxy: fix cookie namespacing

bus-proxy: fix cookie namespacing

If we generate messages from within bus-proxyd, we really must make sure
that we do not clutter the cookie-namespace of our client. The client has
full control over it, so we cannot steal cookies from it. However, we can
re-use the cookie the client used for our request. As long as we only
send a single message, we're fine.