A file called "portable.txt" in the working directory when running any
executable causes the settings path to default to the current working
directory, instead of a system-wide location.

This file can have any contents, or be completely empty. It simple must be
successfully opened.

The config files are then stored in ./${PROGRAM_NAME}_settings.conf - e.g.
"OpenApoc_settings.conf".

This file ensures that files ran as part of the build, or tests invoked from
the buildsystem, default to the portable mode.
