INT 2F - DOS v5.0 TASK SWITCHER - BUILD CALLOUT CHAIN
        AX = 4B01h
        CX:DX -> task switcher entry point (see AX=4B02h)
        ES:BX = 0000h:0000h
Return: ES:BX -> callback info structure (see below) or 0000h:0000h
Notes:  called by the task switcher
        this function is hooked by clients which require notification of task
          switcher activities; the call must first be passed on to the prior
          handler with registers unchanged using a simulated interrupt.  On
          return, the client must build a callback info structure and store
          the returned ES:BX in the "next" field, then return the address of
          its own callback info structure.
        a client program must add itself to the notification chain if it
          provides services to other programs; before terminating, it must
          remove itself from the chain by calling the task switcher's entry
          point with AX=0005h (see AX=4B02h)
        the task switcher entry point should not be saved, as it is subject to
          change and will be provided on any notification call
        the Windows 3.1 Standard Mode supports this API

Format of callback info structure:
Offset  Size    Description
 00h    DWORD   pointer to next callback info structure
 04h    DWORD   pointer to notification function (see below)
 08h    DWORD   reserved
 0Ch    DWORD   address of zero-terminated list of API info structures
                (see AX=4B02h)

Notification function is called with:
        AX = function
            0000h switcher initialization
                Return: AX = 0000h if OK to load
                           = nonzero to abort task switcher
            0001h query suspend
                BX = session ID
                Return: AX = 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 AX=4B02h)
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