Since we have to use pkill, kludgily, for the right combination of
portability and featurefulness, put the code in one place rather than
duplicating it everywhere.
https://bugzilla.gnome.org/show_bug.cgi?id=635089
gnome-volume-control-applet was renamed to gnome-sound-applet when
moved to the control-center module, so we need to check for both names
when identifying the legacy status icon.