* dir.cc: Rename opendir_* to dirent_* throughout.

* dir.cc: Rename opendir_* to dirent_* throughout.
(opendir_states): Move and rename.
* fhandler.h (dirent_states): to here.
* fhandler_disk_file.cc (fhandler_disk_file::readdir): Use raw readdir when
skipping through entries since it is keeping track of "." and "..".
(fhandler_cygdrive::seekdir): Use fhandler_disk_file::readdir to do everything.
* fhandler_virtual.cc (fhandler_virtual::opendir): Set flag indicating that we
provide .  and ..
(fhandler_virtual::seekdir): Ditto.
(fhandler_virtual::rewinddir): Ditto.
* fhandler_registry.cc (fhandler_registry::rewinddir): Ditto.