After creating namespaces we may need to apply some configuration.
For example uid and gid maps should be applied in this moment.

This script should be executed from crtools, but namespaces are created
in a root process, so we need to add a new stage for synchronizing.

v2: use a separate stage instead of socketpair.
Signed-off-by: Andrey Vagin <avagin@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>