INT 21 - DOS 5.0 - GET TRUE VERSION NUMBER
        AX = 3306h
Return: BL = major version
        BH = minor version
        DL = revision (bits 2-0, all others 0)
        DH = version flags
            bit 3: DOS is in ROM
            bit 4: DOS in in HMA
Notes:  this function always returns the true version number, unlike AH=30h,
          whose return value may be changed with SETVER
        because of the conflict from the CBIS PowerLAN redirector (see next
          entry), programs should check whether BH is less than 100 (64h)
          and BL is at least 5 before accepting the returned BX as the true
          version number; however, even this is not entirely reliable when
          that redirector is loaded
        fully reentrant
BUG:    DR-DOS 5.0 and 6.0 return CF set/AX=0001h for INT 21/AH=33h
          subfunctions other than 00h-02h and 05h, while MS-DOS returns AL=FFh
          for invalid subfunctions
SeeAlso: INT 2F/AX=112Fh