dump: use freezer cgroup to seize processes (v4)

Without using a freezer cgroup, we need to do a few iterations to catch
all tasks, because a new tasks can be born. If new tasks appear faster
than criu collects them, criu fails. The freezer cgroup allows to
solve this problem.

We freeze the freezer group, then attaches to tasks with ptrace and thaw
the freezer cgroup. We suppose that all tasks which are going to be
dumped in a specified freezer group.

v2: fix comments from Christopher
Reviewed-by: Christopher Covington <cov@codeaurora.org>

v3: refactor task_seize

v4: fix comments from Pavel

Cc: Christopher Covington <cov@codeaurora.org>
Signed-off-by: Andrey Vagin <avagin@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>