PuTTYgen initial version. Still to do are basic user-friendliness

PuTTYgen initial version. Still to do are basic user-friendliness
features (prompt for passphrase twice, prompt before overwriting a
file, check the key file was actually saved OK), testing of the
generated keys to make sure I got the file format right, and support
for a variable key size. I think what's already here is basically
sound though.

git-svn-id: svn://svn.tartarus.org/sgt/putty@715 cda61777-01e9-0310-a592-d414129be87e