Group the subpipes of upipe_rtp_fec together in the output of upipe_dump()

[JDarnley]

See the attached picture for what I mean. The fec pipes are not grouped or linked together. My request is to make this possible, perhaps like upipe_dup or upipe_ts_demux (not shown here).

(Made with dot -Tpng dump.dot -o dump.png)

While I am using an old version of upipe_rtp_fec I don't see an obvious commit that would change this.