This patch adds two related aspects:
* Incoming packets from own callsign are not parsed (i.e. not added to
the position table / not checked for messages) - fixing #62
* Digipeated packets can get their own notification - probably useful
When a backend does not accept packets immediately after start() (i.e.
because it needs to connect to a server first), start() must return
false and the backend needs to call service.postPosterStarted() as soon
as it is ready to accept data.