Merge branch 't/alsa_improvements'

Was cooking for almost two months.

* t/alsa_improvements:
  alsa: Remove pointless initialization.
  alsa_mix: Allow non-positive mixer values.
  alsa: New writer option: --buffer-time.
  alsa: Work around poll fd problems.
  alsa: Set period time.
  alsa: Also dump hardware params.
  alsa: Improve help text of --device.
  alsa: Improve documentation of struct private_alsa_write_data.
  alsa: Don't set t->error in ->pre_select().