We register Polari as a Telepathy client so mission-control can launch us
to handle channel requests even when not running. However as the specified
command does not actually start the telepathy client, the request will fail
after a timeout in this case.
Fix this by exposing the newly-added 'start-client' action on the command
line and use it to launch the telepathy service.

https://bugzilla.gnome.org/show_bug.cgi?id=770749