AppRun script does not work if call path contains spaces

The AppRun script is responsible for launching portable Linux builds in AppImage, Zero Install, Steam and Itch. If it's call path ($0) contains spaces, it won't find the executable it is supposed to launch.

This is not usually a problem because most of the time, the call path is just ./AppRun; Itch, however, adds " 2" to secondary game installations and then calls AppRun as "...armagetronad 2/AppRun".