[Development report #8] Audio Stack Improvements

Continued work on scrapping the snd_clone device cloning framework and
using DEVFS_CDEVPRIV(9) instead:

- Finalized https://reviews.freebsd.org/D44411. Waiting for more people
  to review and accept the patch.
- Updated sound(4) man page, as well as Wiki page.
- Reviewed https://reviews.freebsd.org/D44469 and used the function in
  D44411.
- Added support for the OSSv4-compatibility alias devices. Disallowed
  access to /dev/dspX.[v|r|vp|vr]Y device. See related discussion in
  previous report, as well as the comments in the review.
- More testing.

Also continued the asynchronous device detach patch:
https://reviews.freebsd.org/D43545

- Updated and simplified diff, now depends on D44411.
- Debugging a kern_poll() panic occuring after hot-unplugging the device
  with jackd running.
- Closed https://reviews.freebsd.org/D11140 since D43545 takes care of
  this change.

Lastly, I gave a talk at AsiaBSDCon in Taipei, Taiwan.

Christos