Links v2: `docker links add`

This includes the patches from #8050 and they need to be merged (And this branch be rebased if necessary) before merging this one. Review should probably also wait since the underlying patch is so large.

This adds `docker links add` according to the proposal @ https://github.com/docker/docker/issues/7468 and a integration-cli test for it.

It does not change the semantics and works almost exactly like `docker run --link` at the moment. I will be providing another patch atop this which will tear down some of our guards against linking to non-running containers, etc.