mixer(8): Improve shorthand volume parsing.

The initial patch had a bug where the full volume syntax, floating point values,
:, + and -, wasn't accepted.

While at it move some defines to enum's.

Fixes: da3d4469ef841be44ac4b346a6b51668d608ed49
Submitted by: christos@
Differential Revision: https://reviews.freebsd.org/D34617
Sponsored by: NVIDIA Networking