Peter Schellenbach's patch: re-implement the PuTTY cryptographic

Peter Schellenbach's patch: re-implement the PuTTY cryptographic
functions as calls to the MS Crypto API. Not integrated into the
Makefile yet, but should eventually allow building of an SSH-enabled
PuTTY which contains no native crypto code, so it can be used
everywhere (and anyone who can get the MS encryption pack can still
use the SSH parts).

git-svn-id: svn://svn.tartarus.org/sgt/putty@425 cda61777-01e9-0310-a592-d414129be87e