[Development report #7] Audio Stack Improvements

Working on scrapping sound(4)'s "snd_clone" device cloning framework and
using DEVFS_CDEVPRIV(9) instead. Such a change will simplify and reduce
code size significantly, as well as improve user-experience. The WIP can
be found on my GitHub branch, but I will submit a patch on Phabricator
ASAP.

https://github.com/christosmarg/freebsd/commit/c8694174e2e9d77ef74c6d8e25d99b2b7d5d5652

- Discussed with Florian Walpen about the patch.
- Reduced overall code size ("7 changed files with 327 additions and
  1,739 deletions" according to GitHub).
- Got rid of all snd_clone* functions, removed clone.* files and related
  sysctls altogether.
- Handled hw.snd.basename_clone to work with the new patch.
- Handle character device destroying. https://reviews.freebsd.org/D43545
  will be rewritten (and end up noticeably smaller/simpler) on top of
  this patch once submitted.
- Cleaned up patch and fixed most bugs I've spotted so far.
- Simplified dsp_open() and dsp_close().
- Removed dsp_cdevinfo* and embedded this information in the cdevpriv(9)
  structure.
- Tested a bunch of different cases. Need to test more and resolve a few
  remaining bugs.

Regarding behavior that is preserved:
- Exclusive access of an audio device still works (i.e VCHANs disabled).
- Multiple processes can read from/write to the device.
- A device can only be opened as many times as the maximum channel
  number (see PCMMAXCLONE in pcm/sound.h).

Behavior changes:
- Only one /dev/dspX device is exposed, as opposed to the current
  /dev/dspX.[X]X devices created by the snd_clone mechanism. The
  user/application now only needs to access the device through /dev/dspX
  or /dev/dsp and sound(4) will take care of all necessary audio
  routing.

Pending work:
- See if it's worth preserving any of the additional devices in
  dsp_cdevs[] (see SND_DSP_DEV attribute in pcm/dsp.c).
- Make sure no OSS IOCTL breaks.
- Add comments to the code.
- Write proper commit message.
- Update man page.
- Update Wiki page once committed.
- As already mentioned, rewrite https://reviews.freebsd.org/D43545.

Additional work outside the cdevpriv(9) patch includes:

Committed snd_hda patch for the	Lenovo Ideapad 330-15 and ThinkPad X230.
https://cgit.freebsd.org/src/commit/?id=5968e477a560406c480d78ebe438a04f57121119

MFC'd all pending patches to stable/14.

Started documenting missing sound(4) sysctls.

Reviewed and committed Florian Walpen's recent snd_uaudio(4) patch.
https://cgit.freebsd.org/src/commit/?id=a9341f0f0ae01b4d249dbf3bacfa420152c46aef

Christos