linuxdvb: add NULL ptr check for frontend on satconf
mpegts: stop network scanning from failing due to bad mux
subscriptions: add extra debug
linuxdvb: add option to stop closing FEs and attempt to stop on retune

Rather than adding an explicit flag (to stop mux calls) on retune I
have moved the frontend close operation to be done in the background
monitor timer. This isn't foolproof but will cover most things
and I can probably add a delay to cause that to not execute immediately
should the timer be about to expire.