epggrab eit: ensure that the grabber always runs

The logic applied breaks several of the grabbers, the idea is a good one but
the implementation is wrong. It's an area that clearly needs thought, but care
must be taken to realise that there are lots of variations in how data is
provided.

The additionm of the user configurable flag on a mux is a good one, that at
least gives the user the option to disable muxes.