Switch channel

Hi Christian,

These commits allow usage of SET_CHANNEL when PVR switch channel. I don't know yet if this method is quicker. But it should be. Also i keep the old method as fallback because SET_CHANNEL cannot to be used in all circonstances. For sample i tested it with my mythtv 0.25 which is installed in a virtual host without input cards; I use demo recorders to test with it and SET_CHANNEL does not run with demo recorder.
Otherwise the commit 0096e87 is required to use SET_CHANNEL, but it would be great to include it anyway. I explain why in the commit.

br.