PC LAN PROGRAM REDIRIFS.EXE internal - Set redirifs entry point

        Entry:
          AX = BF80h
          ES:DI -> FAR entry point to IFS handler in REDIRIFS
        Return:
          AL = FFh if installed
          ES:DI -> internal workspace

        Note:   all future IFS calls to REDIR.SYS are passed to the ES:DI
                entry point