sound: Re-add Acer C720 support with some adjustments

sound: Re-add Acer C720 support with some adjustments

* Add the Acer C720 infrastructure back in.

* Move the core register programming patch into
  gnu/dev/sound/pci/hda/hda_acer_c720_patch.c along with the original
  GPL copyright from the linux file.

  This is the only part of the patch which could reasonably be said to
  have been taken from the linux driver.

* The rest of the patch is basically two things.

  First, mandatory adjustments to the NID routing to detect
  headphone insertion and removal, which was done through trial
  an error with my Acer and then verified against the linux driver.

  Second, we did include the disabling of the remixer based on a
  comment in the linux driver.  Basically NID 11 is disabled, two
  lines of code, and there's only one way to do that.