containers: added pidns/pidns03.c

* Clones a new child process with CLONE_NEWPID flag - the new child
* process mounts procfs to a "proc" directory and checks if it belongs
* to a new pid namespace by:
* 1. reading value of "proc/self", which is symlink
*    to directory named after current pid number
* 2. comparing read value (PID) with "1"

Signed-off-by: Matus Marhefka <mmarhefk@redhat.com>
Signed-off-by: Jiri Jaburek <jjaburek@redhat.com>
Reviewed-by: Jan Stancek <jstancek@redhat.com>
Acked-by: Cyril Hrubis <chrubis@suse.cz>