smtp/imap authentication require md5

[Simon Josefsson]

Do Emacs 21 have a `md5' command?  The md5.el that is floating around
doesn't seem to be copyrighted by FSF, so that may be a problem.

I'd volunteer to write a elisp wrapper around the common "md5sum"
binary if you think it's a good idea to include that.  Or if we can
use the C source from RFC 1321 I could add that to the C level of
Emacs with a elisp interface.  See license below.

Some of Gnus's IMAP/POP authentication methods need md5, and my SMTP
Authentication work for smtpmail.el also need md5.

/* MD5C.C - RSA Data Security, Inc., MD5 message-digest algorithm
 */

/* Copyright (C) 1991-2, RSA Data Security, Inc. Created 1991. All
rights reserved.

License to copy and use this software is granted provided that it
is identified as the "RSA Data Security, Inc. MD5 Message-Digest
Algorithm" in all material mentioning or referencing this software
or this function.

License is also granted to make and use derivative works provided
that such works are identified as "derived from the RSA Data
Security, Inc. MD5 Message-Digest Algorithm" in all material
mentioning or referencing the derived work.

RSA Data Security, Inc. makes no representations concerning either
the merchantability of this software or the suitability of this
software for any particular purpose. It is provided "as is"
without express or implied warranty of any kind.

These notices must be retained in any copies of any part of this
documentation and/or software.
 */