add dummy network strategy

I threw this together for some experiments I'm running locally. Submitting this PR in case it's something you want upstream.

The use case I'm working on now is using link-local services to provide an HTTP service for container introspection similar to EC2's metadata API. I'll post separately about this service if it works out.

Another use case is adding a dummy with a fixed (link-local) address that provides host services, such as an HTTP proxy. The container can be started with a dummy interface then varnish/squid/apache/whatever can be bound to the interface using `ip netns exec` leaving the process invisible to the container but visible on the network with no risk of accidentally linking containers' networks.