INT 2F - DOS v5.0 TASK SWITCHER - FREE SWITCHER ID
        AX = 4B04h
        BX = switcher ID
        ES:DI -> task switcher entry point (see AX=4B02h)
Return: AX = 0000h
        BX = status
            0000h successful
            other error (invalid ID or ID not allocated)
Notes:  called by a task switcher when it exits, unless it was the first loaded
          and is providing the support for AX=4B03h and AX=4B04h
        the task switcher providing the identifiers may call the terminating
          task switcher's entry point as needed
        this call is available from within DOSSHELL even if the task switcher
          is not installed
SeeAlso: AX=4B03h