common: Pass payload to CockpitTransport "control" signal handlers

That way they don't have to reserialize if they want to forward
a message.

Reviewed-by: Marius Vollmer <marius.vollmer@redhat.com>
common: Add cockpit_pipe_transport_get_pipe() function

Reviewed-by: Marius Vollmer <marius.vollmer@redhat.com>
ws: The "logout" message is forwarded

We forward the logout message from the web service to the various
session bridges.

Reviewed-by: Marius Vollmer <marius.vollmer@redhat.com>
bridge: Implement "superuser" channel option

When this is set to true we use pkexec to launch another cockpit-bridge
as root, and run the channel via that bridge.

Closes #1493
Reviewed-by: Marius Vollmer <marius.vollmer@redhat.com>