alsaloop: Fixes and added --workaround option

alsaloop: Fixes and added --workaround option

- added workaround for alsa-lib (pthread configuration parsing issue) -
  the workaround must be activated manually using ('--workaround serialopen')
- fixed avail_min initialization (caused high CPU usage or xruns)
- fixed shared buffer initialization (both capture and playback buffers
  must have equal number of samples in this config)

Signed-off-by: Jaroslav Kysela <perex@perex.cz>