)
Values task switcher notification function is called with:
	AX = function
	    0000h switcher initialization
		Return: AX = status
			    0000h if OK to load
			    nonzero to abort task switcher
	    0001h query suspend
		BX = session ID
		Return: AX = status
			    0000h if OK to switch session
			    0001h if not
	    0002h suspend session
		BX = session ID
		interrupts disabled
		Return: AX = 0000h if OK to switch session
			   = 0001h if not
	    0003h activate session
		BX = session ID
		CX = session status flags
			bit 0: set if first activation of session
			bits 1-15: reserved (0)
		interrupts disabled
		Return: AX = 0000h
	    0004h session active
		BX = session ID
		CX = session status flags
			bit 0: set if first activation of session
			bits 1-15: reserved (0)
		Return: AX = 0000h
	    0005h create session
		BX = session ID
		Return: AX = 0000h if OK to create session
			   = 0001h if not
	    0006h destroy session
		BX = session ID
		Return: AX = 0000h
	    0007h switcher termination
		BX = flags
		    bit 0: set if calling switcher is only switcher loaded
		    bits 1-15: reserved (0)
		Return: AX = 0000h
	ES:DI -> task switcher entry point (see #02819)
Notes:	function 0000h is generally called by the program which controls or
	  invokes the task switcher, rather than by the task switcher itself;
	  the entry point supplied to this function is not necessarily the
	  entry point to the task switcher itself, and may be 0000h:0000h.  If
	  any client indicates that loading is not possible, all clients will
	  be called with function 0007h; thus it is possible for a client to
	  receive a termination notice without a corresponding initialization
	  notice.
	except for functions 0002h and 0003h, the notification handler is
	  called with interrupts enabled and may make any INT 21h function
	  call; interrupts must not be enabled in functions 0002h and 0003h
	function 0007h may be called with ES:DI = 0000h:0000h if the entry
	  point is no longer valid