(struct pending) [st_ino, st_dev]: Remove just-added members.

(struct pending) [st_ino, st_dev]: Remove just-added members.
(queue_directory): Remove just-added 3rd parameter
and associated code, and update callers.
(print_dir): Get dev/inode pair via `fstat (dirfd (reading), ...',
and call visit_dir to detect cycles here.
Push the dev/inode pair onto the new stack.
(dev_ino_obstack): New global.
(DEV_INO_PUSH): New macro.
(dev_ino_pop): New function.
(ASSERT_MATCHING_DEV_INO): New macro.
(visit_dir): Take two parameters (dev,ino), not one `pending'.
(main): Initialized the dev_ino_obstack.
Don't call visit_dir here.
Get the dev/ino pair from the new stack, not from the pending entry.