GH-135: About, not Cancel, shown when we have Help

This, annoyingly, breaks cancelling the dialog with the escape key.
Should this work?  Probably.  Annoying.  Very annoying.  I'm not so
concerned about the problem, as I don't think the help is useful, modern
or actually wanted by most people using it.

Cancellation works by:
 * ssd->cancelbutton->button.iscancel = TRUE;
 * if (ctrl->button.iscancel) actual_base_id = IDCANCEL;
 * When someone presses escape, Windows generates a WM_COMMAND(IDCANCEL)
  * This is not documented anywhere sensible.  Nowhere.  NOWHERE.

Maybe we could hijack the WM_COMMAND before it arrives at the button
handler, or something?  Shrug.