Refactor node_file.cc to not use Promises.

At the same time implement synchronous wrappers of the POSIX functions.
These will be undocumented until we settle on an API. Works like this

    // returns promise as before
    posix.mkdir("test").addCallback(function () {
      sys.puts("done");
    });

    // returns undefined, executed synchronously.
    posix.mkdirSync("test");
    sys.puts("done");

This refactoring is a step towards allowing promises to be implemented
purely in javascript.