Expects: AX    0004H
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: Applications use this fn when they wish to be notified of task-
switcher operations such as switching session out.

To access this fn, fill-in an SwCallbackInfoRec structure and use
a FAR call to the switcher entry address (see INT 2fH 4b02H).
Your program code pointed to by the pfHandler in that structure
will then receive notifications in the form of FAR calls with AX
specifying a notification fn number.

Note: Before terminating your program (for instance, a TSR which
receives notifications) you must use SwSrvcFn 0005H (unhook).

INT 2fH 4bxxH (switcher functions)
- -

SwSrvc Fn 0004H: Hook Notification Chain