pipe: reopen pipes via usernsd

If a pipe is inherited (external), it may be impossible to reopen it
from a restored user namespace due to lack of permession,
so in this case we have to reopen it via usernsd.

https://github.com/opencontainers/runc/issues/1333


Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>
Acked-by: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>