Suppose you're trying to debug why this pipeline doesn't work:
```
$ GST_DEBUG=GST_PIPELINE:DEBUG gst-launch-1.0 \
videotestsrc num-buffers=10 ! x264enc name=enc ! mux.sink_0 \
mpegtsmux name=mux ! fakesink
```
You will encounter this line in the logs:
> gst_parse_perform_link: linking some pad of GstX264Enc named enc to
> pad mux of GstMpegTsMux named mux (0/1) with caps "(NULL)"
It would seem that the element name is being read as a pad name as well,
and that made me wonder if the parsing was not working. However, it was
just a bug in the code printing that log. This patch fixes that bug.
Note that it is possible to specify more than one pad name for each side
of the link. For instance, the following is a valid pipeline that will
remux the video and audio of an MP4 file into MKV:
```
$ GST_DEBUG=GST_PIPELINE:DEBUG gst-launch \
filesrc location=input.mp4 ! qtdemux name=demux \
multiqueue name=mq \
matroskamux name=mux ! filesink location=output.mkv \
demux.video_0,audio_0 ! mq.sink_0,sink_1 \
mq.src_0,src_1 ! mux.video_0,audio_0
```
The new logging accomodates this by using a new utility function to join
strings of pad name lists instead of `PRETTY_PAD_NAME_FMT` (which only
supports one pad name). For example:
> linking pads {video_0, audio_0} of GstQTDemux named demux to pads
> {sink_0, sink_1} of GstMultiQueue named mq with caps "(NULL)"
Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/8415>
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