Use index rather than telldir/seekdir to represent fd_readdir cookie

[CPunisher]

Fixes: nodejs/node#47193

How telldir and seekdir work depends on the operating system. For example, in ubuntu, telldir returns a relative pos of current dirent iterator. But in macos, telldir returns a self-incrementing counter, which is only valid for one DIR*.

The issue is that every single call to fd_readdir opens and closes the dir, so the cookie becomes invalid on the next call, thus getting infinite loop.