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.
