Move functions from copy.c into new modules, since ln needs them, too.

Move functions from copy.c into new modules, since ln needs them, too.

* bootstrap.conf (gnulib_modules): Add file-set.
* gl/lib/file-set.c (record_file, seen_file): Functions from copy.c.
* gl/lib/file-set.h: Add prototypes.
* gl/lib/hash-triple.c (triple_hash, triple_hash_no_name):
(triple_compare, triple_free): Functions from copy.c.
* gl/lib/hash-triple.h (struct F_triple): Define.  From copy.c.
Add prototypes.
* gl/modules/file-set: New module.
* gl/modules/hash-triple: New module.
* src/Makefile.am (copy_sources): New variable.
(ginstall_SOURCES, cp_SOURCES, mv_SOURCES): Use it.
* src/copy.c: Include hash-triple.h.
No longer include hash-pjw.h.
(copy_internal): Don't pass a NULL third argument to record_file,
since that function no longer accepts that.
(record_file): Move this function to file-set.c.
Along the way, remove the code to allow a NULL stat-buffer pointer.
Adjust sole caller.
(seen_file): Move this function to file-set.c.
(struct F_triple): Move declaration to hash-triple.h.
(triple_compare, triple_free, triple_hash, triple_hash_no_name):
Move these functions to hash-triple.c.

Signed-off-by: Jim Meyering <jim@meyering.net>