INT 2F U - DOS 3.3, DOS 5.0 APPEND - HOOK INT 21
        AX = B703h
        ES:DI -> INT 21 handler APPEND should chain to
Return: ES:DI -> APPEND's INT 21 handler
Note:   each invocation of this function toggles a flag which APPEND uses to
          determine whether to chain to the user handler or the original
          INT 21