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

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.

[originally from svn r973]