INT 15 - DESQview - QEXT.SYS - INSTALLATION CHECK
        AX = 11DEh
Return: CF clear if installed
            AX = segment at which QEXT.SYS is located
Note:   a private entry point may be found by searching the beginning of the
          returned segment for the signature string
          "QUARTERDECK EXTENDED MEMORY MANAGER 286"; the word immediately
          prior to the signature contains the QEXT version number in BCD,
          and the word prior to that contains the offset within the QEXT
          code segment of the private entry point

Call private entry point with:
        AH = 00h ???
        AH = nonzero ???