The problem is to get a virtual pid for zombies.
This code gets two list of children from current and target pidns.
Then it removes alive task from the second list and dumps remaining zombies.

A procfs from a target pidns is needed for dumping zombies,
crtools gets it from parasite code.

v2: Fix comment and get proc fd from a root task
Signed-off-by: Andrey Vagin <avagin@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>