Expects: AX    0005H
ES:DI address of an SwCallbackInfoRec structure
Returns: AX    0000H and CF is clear (NC) if successful
CF is set (CY) if the switcher does not support this fn.
Info: Use this fn before terminating an application which has used
SwSrvcFn 0004H (hook notification chain).  This stops the task-
switcher from calling your notification handler.

INT 2fH 4bxxH (switcher functions)
- -

SwSrvc Fn 0005H: Unhook Notification Chain