zdtm: add a small program to create a zdtm container (v2)

I didn't find a way how to do that with help "unshare".
It's simpler to write this program. It looks better than tricks in
zdtm.sh.

v2: proxify exit status

Signed-off-by: Andrey Vagin <avagin@openvz.org>
Acked-by: Serge E. Hallyn <serge.hallyn@ubuntu.com>
Acked-by: Ruslan Kuprieiev <kupruser@gmail.com>
Acked-by: Christopher Covington <cov@codeaurora.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>