Fix string extending

Built-in modules (e.g. the upcoming ustring) should be able to extend
the string metatable, but user modules (and the debug console) should
not be able to.

But currently built-in modules cannot extend the string metatable in
LuaStandalone, while in LuaSandbox the string metatable can be extended
in the console. Fix this and add unit tests.

Change-Id: I15f5598fed318f2fe26b08ec47e16053dddc13c4
Merge "Fix string extending"