Convert du.c to use the functions in lib/hash.c, not private,

Convert du.c to use the functions in lib/hash.c, not private,
slightly-modified copies of those that used to be in cp-hash.c.

(struct entry) [coll_link]: Remove member.
(struct htab): Remove.
(hash_reset, hash_init, hash_insert2, hash_insert): Remove functions.

Include hash.h and same.h.
(htab): Change type of global to `struct hash';
(entry_hash, entry_compare, hash_ins): New functions.
(count_entry): Use hash_ins instead of hash_insert.