Add kernel namespace contexts

Add the following kernel namespace contexts:

  - cgroup_ns
    Cgroup root directory namespace: inode number of the current
    cgroup namespace in the proc filesystem.

  - ipc_ns
    System V IPC, POSIX message queues namespace: inode number of the
    current IPC namespace in the proc filesystem.

  - mnt_ns
    Mount points namespace: inode number of the current mount
    namespace in the proc filesystem.

  - net_ns
    Network devices, stacks, ports namespace: inode number of the
    current network namespace in the proc filesystem.

  - pid_ns
    Process IDs namespace: inode number of the current pid namespace
    in the proc filesystem.

  - user_ns
    User and group IDs namespace: inode number of the current user
    namespace in the proc filesystem.

  - uts_ns
    Hostname and NIS domain name namespace: inode number of the
    current uts namespace in the proc filesystem.

Signed-off-by: Michael Jeanson <mjeanson@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>