INT 2F U - DOS 3+ CRITICAL ERROR HANDLER - INSTALLATION CHECK
        AX = 0500h
Return: AL = 00h not installed, OK to install
             01h not installed, can't install
             FFh installed
Desc:   determine whether a critical error message override is installed
Note:   this set of functions allows a user program to partially or completely
          override the default critical error handler's message in COMMAND.COM
SeeAlso: AH=05h,INT 24

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