Teach PuTTYgen to import from OpenSSH's new key format.

Teach PuTTYgen to import from OpenSSH's new key format.

This is import only, for the moment: I haven't written an exporter
yet. Also, we currently don't support the format's full generality - a
new-style OpenSSH key file can contain multiple keys, but this code
currently only handles files with one key in them. That should be easy
to change, though, given only a little UI.