INT 2F - DR-DOS 6.0 TaskMAX - REGISTER/UNREGISTER TASK MANAGER
        AX = 2704h
        DL = subfunction
            00h unregister task manager
            01h register task manager
Return: DL = status
            00h registered
            01h unregistered
Notes:  a task manager replaces TaskMAX's menu system with its own user
          interface; while one is registered, the TaskMAX hotkeys and
          Ctrl-Alt-Del invoke the manager rather than the built-in menu system
        unregister the task manager before terminating it