network: take explicit ownership of our own interfaces

network: take explicit ownership of our own interfaces

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.