Core: vfs-dir stuff re-implemented

Core: vfs-dir stuff re-implemented

We fist make it simple, as told by hpa. Here're the changes:
1, The DIR structure changed, just a file pointer included.
2. So, the open/close/read-dir stuff in com32/lib/ changed again.
3. We read one stuff from every readdir call.

Signed-off-by: Liu Aleaxander <Aleaxander@gmail.com>