Int 2F Fn B703  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