Set cursor characteristics

        Entry:
          AH = 01h
          CH bits 5-6 = blink attribute
                       (00=normal, 01=invisible, 10=slow, 11=fast)
          CH bits 0-4 = start line for cursor in char. cell
          CL bits 0-4 = end line for cursor in char. cell
        Return:
          AX,SI,DI,BP destroyed by call