Include hash.h.

Include hash.h.
(gnu_list_name): Remove decl.
(struct link): Remove "next" member.
(linklist): Remove.
(start_header): Say "leading `FOO'" rather than "`FOO' prefix" for consistency
with other diagnostics.
(deal_with_sparse): Check for I/O error when closing the file.
(create_archive):
Do not allocate an array of size PATH_MAX, as PATH_MAX might be (size_t) -1.
Instead, allocate an array with the size that's needed.
(hash_link, compare_links): New functions.
(dump_file): Do not exhaust open file descriptors when descending deeply
into a directory, by using savedir rather than opendir/readdir.
Do not zero-fill the name buffer unnecessarily.
Hash the set of links already created, instead of using a linked list.
Fix some bugs in outputting sparse files which caused the sparse tables
to be incorrect.
When a file unexpectedly shrinks, output zeros rather than garbage.
Do not allocate an array of size PATH_MAX, as PATH_MAX might be (size_t) -1.
Instead, allocate an array with the size that's needed.