WIP: User ns prototype for libcontainer.

This is a WIP for adding userns to libcontainer. It was suggested that this be implemented in steps. 
1. Add some configuration to control the feature. It is disabled by default and will be exposed by a future PR.
2. Maps docker-root to root in the container.

Docker-DCO-1.1-Signed-off-by: Mrunal Patel <mrunalp@gmail.com> (github: mrunalp)