Fix contain to work with Linux 3.12 and later

Linux 3.12 introduced tighter restrictions on mounting proc and sysfs,
which prevented contain from correctly mounting /proc and /sys in
containers.

To comply with the new rules, ensure that contain mounts procfs and
sysfs in the new mount namespace before pivoting into the container and
detaching the host root.
Add -c option to contain which disables console emulation
Remove unnecessary 'container' variable from environment of helpers