_O_OBTAIN_DIR flag makes _open use FILE_FLAG_BACKUP_SEMANTICS in
CreateFile call.
FILE_FLAG_BACKUP_SEMANTICS is required to open directory handles.
For ordinary files FILE_FLAG_BACKUP_SEMANTICS overrides file security
checks when the process has SE_BACKUP_NAME and SE_RESTORE_NAME
privileges, see https://learn.microsoft.com/en-us/windows/win32/api/fileapi/nf-fileapi-createfilea
259cd70c1b