INT 21 - DR-DOS 3.41+ - DETERMINE DOS TYPE/GET DR-DOS VERSION
        AX = 4452h ("DR")
        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-user/multiuser nature
                10h single-user
                    AL = operating system version ID (see below)
                14h multiuser
                    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-user capabilities, AX=4451h
          if looking for multiuser; this call should never return multiuser
          values
SeeAlso: AX=4412h,AX=4451h,AX=4459h

Values for 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
 70h    PalmDOS
 71h    DR-DOS 6.0 March 1993 "business update"
 72h    Novell DOS 7.0

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