libmd / md5: Add SHA-512/224.

While there, remove .Tn from man pages.

Also remove an obsolete comment about the 80386.

MFC after: 1 week
Sponsored by: Klara, Inc.
Reviewed by: kevans, allanjude
Differential Revision: https://reviews.freebsd.org/D38373

(cherry picked from commit 2768d7056727c414241ebc4b9d26e62dd5460760)