Freebind testing

To manually validate the IP_FREEBIND behavior in Envoy, you can launch Envoy with freebind.yaml.

The listener free bind behavior can be verified with:

1. envoy -c ./configs/freebind/freebind.yaml -l trace
2. sudo ifconfig lo:1 192.168.42.1/30 up
3. nc -v -l 0.0.0.0 10001

To cleanup run sudo ifconfig lo:1 down.

TODO(htuch): Steps to verify upstream behavior.