Add RootFs field to configuration options in libcontainer's Config

Since currently the container.json file does not include the pathname
to a container's root filesystem, we need to parse /proc/mounts which
is slow and error-prone.  This patch addresses this issue by adding a
new RootFs field.

Signed-off-by: Saied Kazemi <saied@google.com>
Add RootFs field to configuration options in libcontainer's Config

Since currently the container.json file does not include the pathname
to a container's root filesystem, we need to parse /proc/mounts which
is slow and error-prone.  This patch addresses this issue by adding a
new RootFs field.

Signed-off-by: Saied Kazemi <saied@google.com>
Merge branch 'rootfs' of https://github.com/SaiedKazemi/libcontainer into rootfs

Conflicts:
	namespaces/exec.go
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'.