remove prototypes with no matching function and externs with no var
partly checked by millert@

I am retiring my old email address; replace it with my OpenBSD one.

CKSUM_DIGEST_STRING_LENGTH bounding didn't include full range of size_t
and NUL
ok millert krw tb

purge sum and sysvsum code. ok deraadt jmc lteo millert
a few initial leftovers spotted by naddy

Add support for cksum (3 varieties), md4, sha256, sha384, and sha512.
Consequently, cksum moves becomes a link to md5 and moves to /bin.
The cksum man page could use some polishing.