Check if `os.execute` is available before attempting to use it (is not
available on Windows). This is not a real fix as it results in empty
directories being left behind in the computer files. The only actual fix
to this issue is an official `rmdir` Minetest API.
Mod would load but would crash upon giving the computer input. This was a result of the base OS files not being copied meaning that the "code" variable would be nil. Bundled in this commit are other related changes improving the overall filesystem-related stability on Windows. TODO: prevent crash when missing important files like main.lua
