mixer: remove volume backwards compat, add % interpretation

The current situation is fairly confusing, where an integer is interpreted
as a percent until you slap a decimal on it and magically it becomes an
absolute value.

Let's have a flag day in 14.0 and remove this shim entirely. Setting with
percent can still be useful, so allow a trailing '%' to indicate as such.
As a side effect, we tighten down the format allowed in the volume a little
bit by ensuring there's no trailing garbage after the value once it's
separated into left and right components.

Reviewed by: christos, hselasky, pauamma_gundo.com (manpages)
Relnotes: yes
Differential Revision: https://reviews.freebsd.org/D35101