iptables now defaults to the nft backend

The old iptables-nft package name is replaced by iptables, and the legacy backend is available as iptables-legacy.

When switching packages (among iptables-nft, iptables, iptables-legacy), check for .pacsave files in /etc/iptables/ and restore your rules if needed:

• /etc/iptables/iptables.rules.pacsave
• /etc/iptables/ip6tables.rules.pacsave

Most setups should work unchanged, but users relying on uncommon xtables extensions or legacy-only behavior should test carefully and use iptables-legacy if required.