* fhandler.h (dirent_states): Add dirent_isroot, dirent_saw_cygdrive,

* fhandler.h (dirent_states): Add dirent_isroot, dirent_saw_cygdrive,
dirent_saw_dev.
* dir.cc (opendir): Don't zero __flags here.  Push that responsibility to
opendir methods.
(seekdir): Preserve dirent_isrrot in __flags.
(rewinddir): Ditto.
* fhandler_disk_file.cc (fhandler_disk_file::opendir): Set dirent_isroot
appropriately.
(fhandler_disk_file::readdir): Fill in "cygdrive" and "dev" if it is the root
dir and they are missing.
* fhandler_process.cc (fhandler_process::opendir): Set __flags here.
* fhandler_virtual.cc (fhandler_virtual::opendir): Set __flags here.