player: don't remove playback status when reinitializing DVB

Also break that line a bit.
ao_null: properly simulate final chunk, add buffer options

Simulate proper handling of AOPLAY_FINAL_CHUNK. Print when underruns
occur (i.e. running out of data). Add some options that control
simulated buffer and outburst sizes.

All this is useful for debugging and self-documentation. (Note that
ao_null always was supposed to simulate an ideal AO, which is the reason
why it fools people who try to use it for benchmarking video.)
player: write final audo chunk only if there is audio left

Don't call ao_play() if there's nothing left. Of course this still asks
the AO to play internally buffered audio by setting drain=true.
af_lavrresample: set cutoff as double, not int

Regression introduced with commit a89549e8.
stream: split out pthread helper function

Also split the function itself into 3.