INT 21 - DESQview - INSTALLATION CHECK
        AH = 2Bh
        CX = 4445h ('DE')
        DX = 5351h ('SQ')
        AL = subfunction (DV v2.00+)
            01h get version
                Return: BX = version (BH = major, BL = minor)
                Note: early copies of v2.00 return 0002h
            02h get shadow buffer info, and start shadowing
                Return: BH = rows in shadow buffer
                        BL = columns in shadow buffer
                        DX = segment of shadow buffer
            04h get shadow buffer info
                Return: BH = rows in shadow buffer
                        BL = columns in shadow buffer
                        DX = segment of shadow buffer
            05h stop shadowing
Return: AL = FFh if DESQview not installed
Notes:  in DESQview v1.x, there were no subfunctions; this call only identified
          whether or not DESQview was loaded.  DESQview v2.52 performs function
          01h for all subfunction requests 0Ch and higher and appears to ignore
          all lower-numbered functions not listed here.
        DESQview versions 2.5x are part of DESQview/X v1.0x.
BUG:    subfunction 05h does not appear to work correctly in DESQview 2.52
SeeAlso: INT 10/AH=FFh,INT 15/AX=1024h,INT 15/AX=DE30h