When redisplaying the echo area, use the value

When redisplaying the echo area, use the value
of enable-multibyte-characters from when message was called.
(message_enable_multibyte): New variable.
(message2_nolog): Set the variable.
(echo_area_display): Use that variable.
(display_string): New arg MULTIBYTE.  Callers changed.

(display_menu_bar): Include WINDOW_LEFT_MARGIN in maxendcol.