[GSoC'21 Weekly Update #8] Sound mixer improvements

Hello,

Last week I did the following:

sound(4):

- Implemented a new read-only sysctl to report the playback/recording
  mode of the sound device (dev.pcm.X.mode). The sysctl values are:

  1=mixer, 2=play, 4=rec. These values can also be OR'ed together if
  more than one mode is supported.

mixer(3):

- Added functions to make user-defined controls.

mixer(8):

- Made /dev/sndstat "useless" since with the new sysctl we can get all
  the information it provides.

Documentation:

- Worked on the man pages for mixer(3) and mixer(8).

Also on the 21st of July, my mute ioctl patch for sound(4) got merged
into upstream:

https://cgit.freebsd.org/src/commit/?id=0f8dafb45859569aa36b63ca2bb4a1c35c970d1e

The code is available on:
- GitHub: https://github.com/christosmarg/mixer
- Sourcehut: https://git.sr.ht/~crm/mixer
- My Git server: https://git.margiolis.net/mixer/files.html

The project's Wiki article can be found at: 
- https://wiki.freebsd.org/SummerOfCode2021Projects/SoundMixerImprovements