containers: added mountns/mountns03.c

* Tests a slave mount: slave mount is like a shared mount except that
  mount and umount events only propagate towards it

Signed-off-by: Matus Marhefka <mmarhefk@redhat.com>
containers: added mountns/mountns04.c

* Tests an unbindable mount: unbindable mount is an unbindable
  private mount

Signed-off-by: Matus Marhefka <mmarhefk@redhat.com>
Signed-off-by: Cyril Hrubis <chrubis@suse.cz>