Merge pull request #199 from SaiedKazemi/rootfs

Add RootFs field to configuration options in libcontainer's Config
Saturate negative memory stat values at '0'.

Docker-DCO-1.1-Signed-off-by: Vishnu Kannan <vishnuk@google.com> (github: vishh)
Merge pull request #201 from vishh/stats_rounding

Saturate negative memory stat values at '0'.
Add checkpoint/restore functionality to nsinit and libcontainer

New code was added to nsinit and libcontainer to use the criu(8) utility
to checkpoint and restore a container.

While there are no hard-coded assumptions about Docker, the new C/R
functionality has only been tested with Docker containers.

The nsinit command line usage for checkpoint and restore is consistent
with the rest of nsinit commands.  However, for easier C/R usage, the
user can specify the container ID on the command line.  The best way
to get started is to run "nsinit checkpoint --help" and look at the
"EXAMPLE:" section.

Signed-off-by: Saied Kazemi <saied@google.com>