avcodec: set absolute/ephemer comments/values correctly
transcode: give proper src fmt for subtitle overlay
livehttp: add DISCONTINUITY on playlist if we have restarted stream

If provided initial-segment-number option, signal discontinuity so
player knows to reset timestamps etc for new segments.
livehttp: don't remove previous playlist on open if segment-number is given

If we provide initial segment-number, most likely we want to continue
where we have left of, so leave old playlist-file there and just
overwrite it when we updated it.

default initial-segment-number is 1 so don't test against 0