Store relative paths in the backup archive

Originally absolute paths are stored in the backup archive. This makes
it hard to restore a backup to a different folder structure.

This only works if the working directory is the ancestor of all paths, which
is true for the default backupList option.

This is just a quick fix for my personal usage, but i find it annoying that the backup archive stores the absolute paths to the files. I work under Arch Linux, not sure if it is the same under Windows.