play: Shut down alsa on input EOF.

play: Shut down alsa on input EOF.

para_play leaks a lot of memory on exit because we didn't bother
to shut down the alsa subsystem. While this is harmless from the
correctness point of view, it does make it harder to spot real
memory leaks.

Rework the error handling to always shut down alsa via kill_stream().
Combine play_post_monitor() and session_post_monitor() because they
are small enough and the latter was badly named anyway.