Avoid needing blackbox scripts on $PATH

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.

bin/blackbox_cat

@@ -11,7 +11,7 @@ for param in """$@""" ; do
   shreddable=0
   unencrypted_file=$(get_unencrypted_filename "$param")
   if [[ ! -e "$unencrypted_file" ]]; then
-    blackbox_edit_start "$param"
+    "${blackbox_home}/blackbox_edit_start" "$param"
     shreddable=1
   fi
   cat "$unencrypted_file"