WIP Initial attempt at adding Criu support to Docker

[boucher]

This branch represents a merge of current Docker master with the libcontainer criu and swrk branches, along with @SaiedKazemi's original Docker criu branch. In addition to trying to get those branches working, a few features have been added tot he Criu support (in various states of completion), chiefly: being able to specify output directories and being able to leave the process running after checkpointing.

(removed committed vendor code from other branch)

[SaiedKazemi]

@boucher This is great. Thanks for integrating C/R into the latest Docker head. The review process on my part will unfortunately be slow due to other high priority activities and then traveling (vacation/work).