INT 60 - MDEBUG - GET AND SET MDEBUG FLAGS
        AH = 05h
        DS:SI -> password or a null byte
        BL = new value for the semaphor of MDEBUG
             00h  enable popup of MDEBUG
             else disable popup of MDEBUG
Return: AX = return code
            FFFEh password is invalid
            FFFDh display mode is invalid
           else successful
                BL = old value of the semapor of MDEBUG
                BH = old value of the INT 08h semaphor
                    (this semaphor is always reset after this function)