criu: Warn about --namespaces deprecation

The option was in use when we didn't have the /proc/pid/ns links
and thus ns ids. Later we used it at restore time to tell which
namespaces to restore the pstree into. Right now ids are always
generated and this option has no effect (and meaning).

Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>