Install locales in minetest directory, so they are correctly loaded
Don't grant priveleges to non-existent players.

When accidentally misspelling a name, privileges are sometimes granted to non-existent players, leaving them with the extra privileges, but without the default privileges, if they ever join the server. This corrects that by disallowing /grant from working with invalid players. For completeness, it does the same for /revoke.
Don't load menu textures every frame
Add a small optimization to the menu splash and remove a old comment