tvhdhomerun: only run discovery once upon startup (solves #2418)

Bug #2418 revealed that the current discovery code sends datagram broadcasts every 30 seconds. This behavior is pretty unwanted and unnecessary. This PR modifies the discovery code such that it is only run once when starting tvheadend.

An additional feature that would complete this is a discovery button in the web GUI. That is a bit beyond my current grasp of the internals of tvheadend though; maybe someone could help me with that part.