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 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.

This is to enable merging https://github.com/minetest/minetest/issues/6898 post-5.0.0 and is not necessary if that one is merged.