Throughout, remove __d_u.__d_data fields from DIR structure.

Throughout, remove __d_u.__d_data fields from DIR structure.
* include/sys/dirent.h (dirent): Remvoe old_d_ino.
(DIR): Make __d_dirhash a 64 bit value.  Remove __d_data and __d_u.  Add
__flags.
* dir.cc (opendir_states): New enum.
(opendir): Clear new DIR __flags field.
(readdir): Fill in '.' and '..' entries if we hit EOF and we haven't seen them
already.  Nuke setting of old_d_ino.
(rewinddir): Reset DIR __flags field.
(seekdir64): Ditto.
* fhandler_disk_file.cc (fhandler_cygdrive::fhandler_cygdrive): Remove special
handling of "." and ".." since they are now handled automatically.