[GSoC'21 Weekly Update #3] Sound mixer improvements

[Christos Margiolis]

Hello,

This week not much has actually been done because I had to focus a bit
more on my university's exams, but here's the progress I've made.

mixer(8):

- Got rid of the `-r {+|-|^|=}rdev` interface for modifying a recording
  source I implemented last week and brought back the old 
  `{+|-|^|=}rec rdev` interface, since it seems to be more practical.
- I implemented an -o option which is a simpler and modified
  re-implementation of the -s and -S options, but with a few minor
  differences.
- Did some fine-tuning to get the program to work properly in 
  `/etc/rc.d/mixer`.

mixer(3):

- Minor bug fixes mostly.

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

--
Christos