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.
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.
