criu: don't show long usage in case of error

* Don't show long usage in case of usage error, otherwise an actual
  error message will be lost in long output.

* Print error if command is not specified.

* Return 0 if criu --help is used.

Signed-off-by: Kir Kolyshkin <kir@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>