player cleanups:
- exit with 1 upon failures, not 0.
- restrict pcmname and the associated -d cmdline option
  to only oss and alsa builds.
- tidy-up cmdline options and help output.
- fix long version of -d: --device, not --auddev.
- free pcmname if -o option is given
- add missing free for pcmname in audio_open_oss
- do not set the modes array ever frame. set it at each
  song start, and change the relevant digits only upon
  relevant keyboard key hits.
- remove unnecessary initializations from the perc_play,
  pro_mins, pro_secs, apr_mins and apr_secs declarations.
- some other tidy-ups.
cmake'ry: restrict _POSIX_C_SOURCE, _BSD_SOURCE, _XOPEN_SOURCE, _LARGEFILE_SOURCE, _FILE_OFFSET_BITS and _LARGE_FILES defines to unix builds only
openal: fix skips at song start. revise write_output logic.
reduce num_buffers to 8 (was 16 before but only 8 were used.)