this patch add join-ns option to criu CLI and RPC.
This opt can be used in this fomat:
	--join-ns NS:PID|NS_FILE
for example --join-ns net:12345 or --join-ns net:/foo/bar.

pid namespaces is not supported yet. As fork() is needed to make
new pid-namespace work. That makes it hard for criu to track the
child-process through pid because another child process has been
created after fork().
Signed-off-by: Deng Guangxing <dengguangxing@huawei.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>