to require dumping pid namespace. Dump and restore will be failed if
a tress doesn't contain a process init.

pid namespace will be created implicitly if a process init in the tree.

v2: fix comments from Pavel
v3: Restore of pidns should be approved by user
Signed-off-by: Andrey Vagin <avagin@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>