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_shred_all_files

@@ -16,8 +16,7 @@
 # have been decrypted for editing, you will see an empty list.
 
 set -e
-blackbox_home=$( cd "$( dirname "${BASH_SOURCE[0]}" )" && pwd )
-source "${blackbox_home}/_blackbox_common.sh"
+source "${0%/*}/_blackbox_common.sh"
 
 change_to_vcs_root