INT 7F - Alloy MW386 - SET TERMINAL DRIVER FOR ANOTHER USER
        AH = 39h
        AL = new terminal driver number
        DL = user number (FFh = caller)
        DH = machine number if DL <> FFh
Return: CF set if invalid user number
        CF clear if successful
Notes:  only available to supervisors
        the new driver number will not take effect until the user is rebooted