INT 2F - MTEZ XpressFax CASMGR - INSTALLATION CHECK
        AX = CB00h
        BX = 4D53h
        CX = 4949h
Return: AL = status
            00h not installed, OK to install
            01h not installed, not OK to install
            FFh installed
                BX = 6D73h
                CX = 6969h
Note:   this function is equivalent to the standard CASMGR installation check,
          but uses the additional magic values to identify which CAS is
          installed
SeeAlso: AH=2Ah,AX=8000h"FaxBIOS",AX=C000h"MTEZ",AX=CB0Eh,AX=CB16h

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