Loose end from r5031: the Kex panel should only be displayed in

Loose end from r5031: the Kex panel should only be displayed in
mid-session if we are not using SSHv1. I've done this by introducing
a generic `cfg_info' function which every back end can use to
communicate an int's worth of data to setup_config_box; in SSH
that's the protocol version in use, and in everything else it's
currently zero.

git-svn-id: http://svn.tartarus.org/sgt/putty@5040 cda61777-01e9-0310-a592-d414129be87e