Rename 'agent' -> 'bridge' and install to $PATH

This is done for a couple reasons.
    
 1. This really is a bridge into the system, and not an agent doing things on its own. It merely conveys the commands that the web interface sends its way. There is a lot of confusion about what Cockpit does, and this is one step towards clearing that up.
    
 2. The path to the bridge is public API, and will need to be usable across all future versions of Cockpit across different machines. Various distros can't agree on /usr/libexec so we can't really use it in such a public API guarantee.
    
 3. Name is similar to 'systemd-stdio-bridge' which does a similar task, but it much much simpler.