blackbox_edit and blackbox_cat use other blackbox scripts internally,
but assume that they will be found on $PATH. In testing it's useful to
be able to run these scripts by pathname without first putting them on
$PATH, and all the other scripts work just fine in these circumstances.
This edit fixes this by prefixing all references to other scripts in
blackbox_edit and blackbox_cat with ${blackbox_home}, which is conveniently
set as part of sourcing _blackbox_common.sh.
b35c09609b