Epggrab changes

[perexg]

This is a set of epggrab changes trying to solve issues:

1. OTA epggrab - do not tune to muxes with unwanted epg information (no channels assigned)
2. internal epggrab - use multi-line cron description for times to activate grabbing instead the simple interval
3. OTA epggrab - use multi-line cron description for times to activate grabbing instead the build-in interval
4. OTA epggrab - make the OTA mux epggrab timeout configurable in webui
5. MPEGTS MUX config - allow to force the EPG scan per mux including the provider selection

UPDATE: The 3) and 4) notes were implemented.

[perexg]

@adamsutton : I am finished. Any objections against the pushing this to master ?

[adamsutton]

@perexg on first glance looks fine, but I'd like to look at it in a bit more detail. The one thing I would like to avoid is the in place config migration. This has been the approach in the past, and for small stuff is fine. But there is now a "general" method for migrating configuration, if you look in src/config.c

I admit I mainly added it for handling the really big shifts of the DVB changes or stuff that is just not feasible to do in-line. But I think it helps to keep the normal code cleaner if we do this stuff elsewhere.

Ultimately we can "remove" that stuff in the future if we ever decide that version X is simply too old to bother with.

[perexg]

@adamsutton : Added the config migration, but I have not tested it yet.. There is a lot of work - especially for old xmltv config migration.. It would be nice to review it..