Update libcontainer to 1597c68f7b941fd97881155d7f077852e2914e7b

This commit contains changes for docker:
* user.GetGroupFile to user.GetGroupPath docker/libcontainer#301
* Add systemd support for OOM docker/libcontainer#307
* Support for custom namespaces docker/libcontainer#279, docker/libcontainer#312
* Fixes #9699 docker/libcontainer#308

I put all in one commit, because don't want to have broken commits in master for better git-bisect experience.

ping @icecrime @erikh @jfrazelle @unclejack @crosbymichael @tiborvass