Always setting BLACKBOX_HOME

This makes the beginning of all files the same and a little simpler.

`${0%/*}` turns "/home/user/repository/bin/blackbox_edit" into
"/home/user/repository/bin", exactly like basename but without eating a
process.

Because other scripts needed `$blackbox_home` I made this into a
standardard variable that's always available.

This also loads _stack_lib.sh always because _blackbox_common.sh
requires it.

bin/blackbox_cat

@@ -4,14 +4,13 @@
 # blackbox_cat -- Decrypt a file, cat it, shred it
 #
 set -e
-blackbox_home=$( cd "$( dirname "${BASH_SOURCE[0]}" )" && pwd )
-source "${blackbox_home}/_blackbox_common.sh"
+source "${0%/*}/_blackbox_common.sh"
 
 for param in "$@" ; do
   shreddable=0
   unencrypted_file=$(get_unencrypted_filename "$param")
   if [[ ! -e "$unencrypted_file" ]]; then
-    "${blackbox_home}/blackbox_edit_start" "$param"
+    "${BLACKBOX_HOME}/blackbox_edit_start" "$param"
     shreddable=1
   fi
   cat "$unencrypted_file"