mixer .8: Clean up the manual page

mixer .8: Clean up the manual page

- Use correct macros (e.g., Pa for paths, Ar for arguments, Cm for
  command modifiers).
- Pet igor and mandoc -Tlint (e.g., start sentences after a newline).
- Use Ta instead of a tab character in tables.
- Stylize all table headers with Sy consistency.
- Add a missing "vol" variant to the synopsis of "dev.volume".
- Sort dev.recsrc command modifiers consistency.
- Use "Bd -literal" for code blocks in the examples. "Bl -tag" is not
  the right macro for that.

Fixes:	903873ce1560 Implement and use new mixer(3) library for FreeBSD.