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_common.sh

@@ -7,7 +7,13 @@
 # Usage:
 #
 #   set -e
-#   . _blackbox_common.sh
+#   source "${0%/*}/_blackbox_common.sh"
+
+# Load additional useful functions
+source "${0%/*}"/_stack_lib.sh
+
+# Where are we?
+: "${BLACKBOX_HOME:=$(cd "${0%/*}" ; pwd)}" ;
 
 # Where in the VCS repo should the blackbox data be found?
 : "${BLACKBOXDATA:=keyrings/live}" ;   # If BLACKBOXDATA not set, set it.