Add mod security

This preserves compatibility with most mods.

Things that you can't do anymore:
  * Access any files not in the mod, world, user, or share path.
  * Run Lua bytecode.
  * Use `require()` or anything in the `package` library (`require()` and some `package` functions can load C modules, the rest are only useNote:
This isn't quite ready yet, you can still load bytecode if `init.lua` is a bytecode file.  ful if you have access to the other functions).
  * Use `os.execute()` or `io.popen()` to run shell commands.
  * Use the registry or get/set user values with the debug library (user values are a Lua 5.2 feature anyway).

You can do just about anything else, including using `getfenv` / `setfenv` and `getmetatable` / `setmetatable` (these functions are only dangerous if the sandbox is created from Lua).

I'd appreciate any comments on other possible security holes.

What's the point of this, what makes it safe? Is there any reason bytecode is prohibited?