Don't daemonise the listener thread when actually a service

If we do this and then don't do the readline trick then the python
process quits before anything has had a chance to happen. It's nice
to be able to Ctrl+C the process when developing, but as noted
previously this isn't viable when running as a real service.

Change-Id: Idda9aa6ccac6bdd06926197be976a99a77652c29