mpegts: ensure mux is stopped if force is set
mpegts: fix service enumeration, failing to store weight

This caused oscillation between competing subscriptions!
subscription: Created a proper mux subscription mechanism

It's a bit ugly to link subs and mpegts directly, but its not the end
of the world (and probably unavoidable without lots of duplication).

I'm still not convinced its robust to mux deletions if subs exist on
that mux. Probably needs mmi to be ref counted.

There is also a special kind of sub that expects to receive no stream
data, i.e. all data will come from the SI tables only. This special
sub is automatically unsubscribed (but that might need changing).
epggrab: updated to the new mux sub API
webui: make it possible to subscribe to a mux

I'm not entirely convinced this is fully working. I'm sure the data
rate was too low.
subscription: some minor corrections for mux sub
linuxdvb: fixed leaving dangling file handles and stop FE on stop

Note: really we don't want to close the FE if we're planning to
retune, but I need to change the API to support that operation.