Add userspace namespace contexts

Add a context for each available kernel namespace which currently are :
cgroup, ipc, mnt, net, pid, user and uts. The id chosen to identify the
namespaces is the inode number of the file representing each one of them
in the proc filesystem.

This was introduced in kernel v3.8.0, if any of these context are
enabled on a system running an older kernel, zero will be returned.

Signed-off-by: Michael Jeanson <mjeanson@efficios.com>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>