ws: Rename error code 'no-agent' to 'no-cockpit'

And update its message, since it was a bit confusing.

Reviewed-by: Marius Vollmer <marius.vollmer@redhat.com>
ws: Rename cockpit_ws_agent_timeout to cockpit_ws_session_timeout

That's really what it's watching, various sessions and is related
to the CockpitSession structs that live in cockpitwebservice.c

Reviewed-by: Marius Vollmer <marius.vollmer@redhat.com>
bridge: 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.

Reviewed-by: Marius Vollmer <marius.vollmer@redhat.com>
man: Cleanup manual building automake foo

Reviewed-by: Marius Vollmer <marius.vollmer@redhat.com>
man: Add manual page for 'cockpit-bridge'

Closes #1349
Reviewed-by: Marius Vollmer <marius.vollmer@redhat.com>