tsmux: When selecting random PIDs, name the pads according to those PIDs

Some elements will make use of the automatically generated names to
create new pads in future muxer instances, for example splitmuxsink.

Previously we would've created a pad with a random pid that would become
"sink_0", and then on a new muxer instance a pad "sink_0" and tsmux
would've then failed because 0 is not a valid PID.

Part-of: <gstreamer/gst-plugins-bad!2358>
29 jobs for !2358 with backport-mr-2318-into-1.18-tsmux-pad-name-with-pid in 26 minutes and 2 seconds (queued for 4 seconds)
merge request