Preliminary support for RSA user authentication in SSH2! Most of the

error messages are currently wrong, and Pageant doesn't yet support
the new key type, and I haven't thoroughly tested that falling back
to password authentication and trying invalid keys etc all work. But
what I have here has successfully performed a public key
authentication, so it's working to at least some extent.

git-svn-id: svn://svn.tartarus.org/sgt/putty@973 cda61777-01e9-0310-a592-d414129be87e