[mythtv] EIT active scanner

[Yeasah Pell]

Some providers aggregate EIT for multiple transports onto a single 
transport, so that only one out of a bunch of multiplexes will have EIT 
data on it (but it will apply to the whole group of them)

Currently the active EIT scanner assumes that any multiplex with a 
channel that uses EIT will have EIT available, which can result in a lot 
of useless tuning activity (and potentially more rotor activity than is 
needed)

Does adding a field to the dtv_multiplex table that indicates the 
presence of EIT seem like a good idea? This way we could watch for EIT 
at scan time, and populate this field accordingly, and the active 
scanner could use this information to determine that there's no point in 
scanning a particular multiplex (since it isn't going to find any EIT there)

The only way I figure it can go wrong is if a provider doesn't transmit 
EIT 100% of the time and a multiplex could be marked incorrectly as 
non-EIT at scan time -- it seems kind of unlikely that a provider would 
shut EIT off for an entire transport for certain times of the day, but 
you just never know what horrible things broadcasters will do. If that's 
the case, we could still collect the EIT presence information, but only 
optionally use it to limit EIT scans (i.e. as a per-input DVB option 
like the DVB radio checkbox, etc.)