#3 Consider "false" and "nil" the only false values in load_mod_xxx

This will pave the way to implementing mod paths in future, to fix [4183](https://github.com/minetest/minetest/issues/4183) without breaking the sharing of world.mt files with versions that don't support mod paths, because when "true" is replaced with a mod path, it will still be considered true.

Merged, closing