Problem:    The first ":echo 'hello'" command output doesn't show.  Mapping
            for <S-F3> gets triggered during startup.
Solution:   Add debugging code for the termresponse.  When receiving the "Co"
            entry and when setting 'ambiwidth' redraw right away if possible.
            Add redraw_asap().  Don't set 'ambiwidth' if it already had the
            right value.  Do the 'ambiwidth' check in the second row to avoid
            confusion with <S-F3>.