Remove userland proxy

This builds up on @phemmer work and comments (#6810, [`libcontainer's` #62](https://github.com/docker/libcontainer/pull/62)).

A few notes on the implementation:

- A new `--route-localnet` daemon flag was added to control the `net.ipv4.route_localnet` flag: it is *very* succinctly documented, so maybe one of you will want to come up with a nice explanation paragraph for `docs/sources/articles/networking.md`.
- The new [iptable `POSTROUTING` rule](https://github.com/docker/docker/pull/6810#issuecomment-49370841) is not depending on any command line flag (I don't believe it has to be).
- Integration tests cover both container -> container communication through the host IP and host -> container communication through localhost.