Extended control-break checking (INT 21h, 33h)

        Entry:
          AH = 33h
          AL = subfunction
                 00h Get State
                 01h Set (DL = 0 for OFF or 1 for ON)
                 02h internal, called by PRINT.COM (DOS 3.1)
                 05h Get Boot Drive (DOS 4.00)

        Return:
          DL = current BREAK setting if AL = 00h
                  0 BREAK=OFF
                  1 BREAK=ON
             = Boot drive (01=1, 03=C, ...)
          AL = FFh if error