updated md5 library

[Luiz Henrique de Figueiredo <lhf@...>]

I've updated my MD5 library. It's available as usual at
	http://www.tecgraf.puc-rio.br/~lhf/ftp/lua/

The main change is that the "digest" method can now be called at any point,
without disrupting the process. This was correctly mentioned as an issue
in the nice LuaCheia documentation for this library:
	http://luacheia.lua-users.org/doc/md5.html

The solution was pretty simple: just copy the MD5 context before making the
digest. I saw this solution in the Python MD5 module.

I've also made small changes in the Makefile (e.g., -lmd5 works in RH9).

Enjoy.
--lhf