Fix gapless playback (broken by playback.c changes in 3.4).  Closes: #314.
Implement default pause(), seek(), and stop() functions in core so they
do not need to be copied in each input plugin.  Add check_stop() and
check_seek() functions to InputPlayback.  Also, input plugins no longer
need to call op->pause() and op->flush() at playback start; it is done
in op->open_audio().