Update iptables systemd unit for new version of iptables

It appears in the latest version of iptables imported from Debian, there are new two forms of "iptables-restore": iptables-nft-restore and iptables-legacy-restore. Since we are using older style rules, switching to iptables-legacy-restore (and iptables-legacy-save, also for the IPv6 variants).

These also seem to be in /usr/sbin/, not /sbin/, so updating to take that into account too.

Signed-off-by: Martyn Welch martyn.welch@collabora.com