INT 21 - DR-DOS 3.41+ - DETERMINE DOS TYPE/GET DR-DOS VERSION
        AX = 4452h
        CF set
Return: CF set if not DR-DOS
            AX = error code (see AH=59h)
        CF clear if DR-DOS
            DX = AX = version code
            AH = single-tasking/multitasking
                10h single-tasking
                    AL = operating system version ID
                        60h DOS Plus
                        63h DR-DOS 3.41
                        64h DR-DOS 3.42
                        65h DR-DOS 5.00
                        67h DR-DOS 6.00
                14h multitasking
                    AL = operating system version ID (see AX=4451h)
Notes:  the DR-DOS version is stored in the environment variable VER
        use this function if looking for single-tasking capabilities, AX=4451h
          if looking for multitasking; this call should never return multi-
          tasking values
SeeAlso: AX=4451h,AX=4459h