tar: don't assume size of a sparse file chunk fits in size_t

* src/tar.h (struct sp_array): Change numbytes from size_t to off_t.
All uses changed.
* scripts/xsparse.c (struct sp_array): Likewise.
Include <stdint.h>, for SIZE_MAX.
(expand_sparse): Don't try to allocate a buffer bigger than
SIZE_MAX bytes.
* src/common.h (SIZE_TO_CHARS, size_to_chars, SIZE_FROM_HEADER):
(size_from_header): Remove decls.
* src/create.c (size_to_chars): Remove.
* src/list.c (size_from_header): Remove.
* src/sparse.c (sparse_extract_region, check_data_region):
(oldgnu_add_sparse, oldgnu_store_sparse_info, pax_decode_header):
Don't assume chunk sizes fit in size_t.
(oldgnu_add_sparse): Check for off_t overflow.
* src/xheader.c (sparse_numbytes_decoder, sparse_map_decoder):
Likewise.

src/tar.h

@@ -265,7 +265,7 @@ enum archive_format
 struct sp_array
 {
   off_t offset;
-  size_t numbytes;
+  off_t numbytes;
 };
 
 struct xheader