Rewrite of the set-volume helper script

Cleaned up code, removed dependency on potentially defunct 'osd-text'
script, which may need to be revisited, to make osd_cat output more
uniform for any other potential applications.

Originally I planned to control mpd / mpc with osd-mpd, but I don't
really use it, and that was the other case for 'standardizing' output
from osd_cat.
Remove osd-mpd and osd-text