package/iptables: S35iptables script cleanups

This patch removes unneeded redirections when
calling iptables-restore and iptables-save, and
it adds the save operation to the usage help.

Signed-off-by: José Pekkarinen <jose.pekkarinen@unikie.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>