disable potentially harmfull lua modules and replace builtin usecases

Security isn't a goal of minetest as I have learned.
Maybe there are still some people out there who want at least minimal protection for their server against malicious mods. This patch shows how security critical functions can be disabled (write to any file, execute any application ... )

Be warned mods using this functions won't work anymore.

This patch disables possibility for mods to store data to server filesystem, therefore minetest needs a api to save mod data to a world. This will be subject of another patch I'm gonna write soon.