Add support for EIT data post-processing with an external program

This PR adds a new flavour of the EIT OTA Grabber that pushes the EIT events to a comet mailbox instead of updating the EPG directly.

Background: I am currently working on EPG support for Germany. Sadly, there is neither a decent EPG provider for end users in Germany (the only one I know of is epgdata.com which is both commercial and unreliable) nor a working XMLTV grabber, so over here we're stuck with what we can get from EIT.

Fortunately, available EIT data (at least on DVB-T) varies from somewhat useable to pretty good. Some providers carry all information for a decent EPG (including series linking), most others have at least enough information for looking up series/episodes on thetvdb.com and movies on themoviedb.org. 

My current plan is to use the new interface added in this PR to push EIT events to an external program that massages the EIT data into EPG info, enhances it with external data, and then sends the finished EPG entries back to tvheadend over the existing "External interface" sockets. 

I'm going with an external program because I want to keep provider or channel specific post-processing out of tvheadend itself and because I don't think it's a good idea to have (possibly slow) requests to external sources like thetvdb.org in the EIT grabber code path.