For all other tasks only unsed service descriptors will be closed.

This change allows to have file descriptors, which may be used for
restoring namespaces. All non-server descriptors must be closed before
restoring files.
Signed-off-by: Andrey Vagin <avagin@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>