Support fs.open() flags being a number vs. string

Hi everyone!

I'm trying to do this and I'm getting flags is not valid:

```js
fs.open("/file.txt", fs.constants.O_RDWR | fs.constants.O_CREAT, function(err, fd) {
  if(err) throw err;
  fs.fstat(fd, function(err, stats) {
    if(err) throw err;
    fs.close(fd);
  });
});
```