digests, copy-file: increase the IO buffer size from 4KiB to 32KiB

digests, copy-file: increase the IO buffer size from 4KiB to 32KiB

This results in a significant decrease in syscall overhead
giving a 3% speedup to the digest utilities for example
(when processing large files from cache).
Storage is moved from the stack to the heap as some
threaded environments for example can have small stacks.

* lib/copy-file.c (copy_file_preserving): Use a 32KiB malloced buffer
* modules/copy-file: Depend on xalloc
* lib/md2.c: Likewise
* lib/md4.c: Likewise
* lib/md5.c: Likewise
* lib/sha1.c: Likewise
* lib/sha256.c: Likewise
* lib/sha512.c: Likewise