Read cursor position

        Entry:
          AH = 03h
          BH = Page number
                 0   in graphics modes
                 0-3 in modes 2&3
                 0-7 in modes 0&1
        Return:
          DH = row
          DL = column
          CH = cursor start line
          CL = cursor end line
          AX,SI,DI,BP destroyed by call