mixer(8): Improve mute and recsrc controls

[christos]

Summary

The input options of "dev.mute" (+, -, ^) and "dev.recsrc" (+, -, ^, =)
are quite cryptic. Allow the input to also be an actual description of
what these options do.

+ -> add (recsrc)
- -> remove (recsrc)
^ -> toggle (recsrc, mute)
= -> set (recsrc)
0 -> off (mute)
1 -> on (mute)

Also, deprecate the use of the symbol options in the EXAMPLES section of
the man page, by using the new descriptive options.

In the future, we might want to get rid of the symbol options
altogether, but preserve backwards compatibility for now.

Sponsored by:	The FreeBSD Foundation
MFC after:	2 weeks

Test Plan

christos@tpad$ mixer vol.mute=toggle
vol.mute: on -> off
pcm0:mixer: <Realtek ALC257 (Analog 2.0+HP/2.0)> on hdaa0  (play/rec) (default)
    vol       = 0.85:0.85     pbk
    pcm       = 1.00:1.00     pbk
    speaker   = 1.00:1.00     pbk
    rec       = 0.37:0.37     pbk
    ogain     = 1.00:1.00     pbk
    monitor   = 0.67:0.67     rec src

[christos]

Remove the case statements altogether and test the symbol options after the
descriptive one.