Thread library: support for an atomic flag
Audio output thread is killable
Asynchronous_audio: perform sine sweep
Fixed unused variable warning
Asynchronous_audio: kill thread and pause for 2 seconds

This is a test to make sure that it dies.
It is now possible to have mixahead of 0
Implementation of audio output V1
Merge branch 'audio_output'

This includes all infrastructure but no actual public api; and there may be efficiency issues.
- It uses too many locks for my liking.
- It does not yet do anything but default audio devices.
- It does not yet properly clean itself up.
Merge branch 'master' of github.com:camlorn/libaudioverse