fs: make callbacks run in global context

Callbacks that were passed to the binding layer ran in the context of the
(internal) binding object. Make sure they run in the global context.

Before:

  fs.symlink('a', 'b', function() {
    console.log(this); // prints "{ oncomplete: [Function] }"
  });

After:

  fs.symlink('a', 'b', function() {
    console.log(this); // prints "{ <global object> }"
  });