relative path for /USE option. Refs: #94.
As proposed by luowy.
A relative path for the /USE option supports the following use case:
1. Prepare a script for starting BlackBox with the /USE "." option.
2. Copy this script into a target directory.
3. Run the script from the target directory and thereby use this directory in the /USE option
and as start directory for the file open dialog.
(This is exactly the use case needed for working on issues of the BlackBox framework center.)

Signed-off-by: Josef Templ <josef.templ@gmail.com>