DTVRecorder: buffer up each payload until we know if the payload is a
keyframe.

Especially with H.264, we can be several packets into a payload before we
know if it is a keyframe or just a frame or something else.  Always buffer
the packets of a payload until we know what we are doing with the payload.

If we need to switch ringbuffers, we now do it so the new file will start at
the beginning of the payload holding a keyframe.  Each file will also start
with a PAT/PMT if the "recorder" calls for it.