Target leading slash checks

Replaces this with a case statement rather than a regular bracketed if
check, with the former being portable and the latter not.
Target 'addons' substring check

Also replaces this with a case block to be interpreted correctly on
regular /bin/sh.
Missing MP3 file during installation/packaging

Both the install and package script should include the MP3 file from the
source directory.

This fix must be backported via a cherry-pick to the pre/v1.0.0 branch.
Switch packager to check for zip extension later

This brings this script in-line with the others in that it will now
check for the zip extension only after the existence of the parameter
has been checked.