Embed a DES implementation rather than relying on lib(x)crypt

[zackw]

Fixes #5 and #6 (the latter appears to be a duplicate). Also eliminates the dependency on libcrypto (the embedded MD4 implementation is used unconditionally), cleans up the code a little and the Makefiles a whole lot.

I copied the DES implementation over from libxcrypt, since that's the one that I have conveniently to hand. It was originally written for NetBSD and is therefore 3-clause BSD licensed, which I hope is OK.

Note: this compiles cleanly on one computer but has not been tested at all beyond that.