INT 21 - DR-DOS 5+ - DETERMINE DOS TYPE
        AX = 4412h
        CF set
Return: CF set if not DR-DOS
            AX = error code (see AH=59h)
        CF clear if DR-DOS
            DX = AX = version code
                1060h = ???
                1063h = DR-DOS 3.41 ???
                1065h = DR-DOS 5.0
                1067h = DR-DOS 6.0
Note:   this call is identical to AX=4452h