Display character on status line

        Entry:
          AX = DE0Ah
          BL = character

        Return:
          Character displayed, next call will display in next position
          (which wraps back to the start of the line if off the right
          edge of screen)

        Notes:  Displays character on bottom line of *physical* screen,
                regardless of current size of window (even entirely hidden)
                Does not know about graphics display modes, just pokes the
                characters into display memory