Backports for 0.9.1

This PR backports the following PRs to 0.9 for 0.9.1:
https://github.com/dotcloud/docker/pull/4387
https://github.com/dotcloud/docker/pull/4731
https://github.com/dotcloud/docker/pull/4648
https://github.com/dotcloud/docker/pull/4684
https://github.com/dotcloud/docker/pull/4782