nsresourced: explicitly remove network interfaces when their userns goes away

nsresourced: explicitly remove network interfaces when their userns goes away

Let's tighten the screws a bit on the network interfaces we delegate,
and explicitly destroy them, just like we destroy delegated cgroups.

Ideally, this should happen automatically because the userns goes away
that pins the veth, or because the client holding an fd for a tap device
goes away as the userns goes away. But you never know who keeps a
reference, hence let's explicitly destroy this too.