parasite: get a file descriptor for procfs from a target pidns (v2)

It will be used for dumping zombies.

v2: Add a fast path, if /proc belongs to the target pipns. it's usually for CT.

Signed-off-by: Andrey Vagin <avagin@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>