MD4 checksum fix

[cbarratt at users.sourceforge.net]

> It looks like it will work OK, but it's kinda ugly in that starts
> embedding version stuff into the mdfour implementation. Still... its
> better than the nothing I've produced :-)

Yes, it's certainly not elegant.  An alternative would be to
have two different sets of MD4 routines and then have more
logic in checksum.c to call the correct routines.  That makes
for more changes to checksum.c but avoids a library depending
upon a global variable.

Or we could add a new function in mdfour.c, eg: mdfour_broken(),
that gets called once at startup if remote_version < 27.  This
would avoid the need for md4 to pull in remote_version.

Craig