CA config: make the 'Done' button cancel and not default.

This means that, on the one hand, an absentminded press of Return
doesn't dismiss the entire CA config box, which would be pretty
annoying if you were half way through entering a load of fiddly stuff.

And on the other hand, you _can_ press Escape to dismiss the box,
which is less likely to happen by accident.

ssh/ca-config.c

@@ -346,7 +346,7 @@ void setup_ca_config_box(struct controlbox *b)
     ctrl_columns(s, 5, 20, 20, 20, 20, 20);
     c = ctrl_pushbutton(s, "Done", 'o', HELPCTX(no_help),
                         ca_ok_handler, P(st));
-    c->button.isdefault = true;
+    c->button.iscancel = true;
     c->column = 4;
 
     /* Load/save box, as similar as possible to the main saved sessions one */