Start of an SSH-server-specific config structure.

This is much simpler than Conf, because I don't expect to have to copy
it around, load or save it to disk (or the Windows registry), or
serialise it between processes. So it can be a straightforward struct.

As yet there's nothing actually _in_ it. I've just created the
structure and arranged to pass it through to all the SSH layers. But
now it's here, it will be a place I can add configuration items as I
find I need them.

ssh.c

@@ -283,7 +283,7 @@ static void ssh_got_ssh_version(struct ssh_version_receiver *rcv,
 #else
                 NULL,
 #endif
-                &ssh->stats, transport_child_layer, false);
+                &ssh->stats, transport_child_layer, NULL);
             ssh_connect_ppl(ssh, ssh->base_layer);
 
             if (userauth_layer)