Make ls -R detect directory cycles.

Make ls -R detect directory cycles.

Include hash.h, same.h, and xalloc.h.
(INITIAL_TABLE_SIZE, LOOP_DETECT): Define.
(active_dir_set): New global.
(struct dev_ino): Declare.
(dev_ino_hash, dev_ino_compare, dev_ino_free): New functions.
(visit_dir, free_pending_ent): New functions.
(main): Initialize the active_dir_set hash table, if necessary.
Don't confuse a marker entry with a real one.
Detect loops.
Manage the set of active directories.
Free the hash table.
(queue_directory): Add a new parameter.
Ensure that we set the new dev/ino members for each enqueued directory.
Update all callers.
(print_dir): Don't confuse a marker entry with a real one.
(extract_dirs_from_files): Insert a marker entry before inserting
the entries for subdirectories.