MFC r269228:
Add support for SOUND_MIXER_INFO IOCTL, used by gstreamer.

Submitted by:   Dmitry Luhtionov <dmitryluhtionov@gmail.com>
Approved by:	re (marius)
MFC r270423:
Restore pre-r239157 handling of sched_yield(), when thread time slice
was aborted, allowing other threads to run.  Without this change thread
is just rescheduled again, that was illustrated by provided test tool.

PR:		192926
Submitted by:	eric@vangyzen.net
Approved by:	re (marius)