service: handle input priority better

Ensure, that inputs with higher priorities are preferred.
The decision line is:

1) re-use input if the in-use mux matches
2) use a free input with highest priority
3) cancel the mux/input with smallest weight and use it for new
recode mpegts_mux_start() to follow rules as service_find_instance()
mpegts: Add extra weight for inputs with more subscribers

This simple code assures that the inputs having more subscribers
will be selected after ones with less subscribers.
service: find the tuner/source with the lowest weight for the override action
mpegts_mux_start: follow weight handling as for service, fix errors