Spotted randomly when going through CI logs:
systemd-udevd[658]: /usr/lib/systemd/network/80-6rd-tunnel.link:21: Unknown section 'Network'. Ignoring.
Follow-up for 658169e6d3.
This is a follow-up for #30786 and uses it to assign
ID_NET_MANAGED_BY=io.systemd.Network to all all network interfaces that
we consider ours to manage. This should hopefully have the effect that
other well-behaving managers won't fight for these devices.
This doesn't bother with network interfaces we match inside containers,
since udev is not available there anyway.