mpegts network_scan: fix problems with having multiple networks

previously as soon as any mux failed it would give up assuming nothing else
could be tuned. However just because no tuner was available that could service
that mux doesn't mean there wasn't one that couldn't service another.

The only real issue I can see with this is if we're idle scanning it's possible
due to bad luck to starve muxes of an attempt. We can either rely on random
chance or put some additional weighting based on time? Though that could be
problematic for other reasons.