Bug 927 - Multithreading (MT) issues with ALAudioSink (openal-soft)

- Using update openal-soft (commit 7297c3214a4c648aaee81a9877da15b88f798197)

- Analyzed openal-soft threading issues:
  - a global-lock would have removed the issue

  - turns out that using ALC_EXT_thread_local_context's alcSetThreadContext(..)
    instead of alcMakeContextCurrent(..) solves the issue

- Cleaned up al*GetError() queries and handling

- Simplified flush/dequeue buffers
doc/HowToBuild.html: Complete Debian package list, add note on multiarch packages