mixer(8): Improve mute and recsrc controls

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
Reviewed by: dev_submerge.ch, imp
Differential Revision: https://reviews.freebsd.org/D43796