network: firewall integration with NFT sets

New directives `NFTSet=`, `IPv4NFTSet=` and `IPv6NFTSet=` provide a method for
integrating configuration of dynamic networks into firewall rules with NFT
sets.

/etc/systemd/network/eth.network
```
[DHCPv4]
...
NFTSet=netdev:filter:eth_ipv4_address
```

```
table netdev filter {
        set eth_ipv4_address {
                type ipv4_addr
                flags interval
        }
        chain eth_ingress {
                type filter hook ingress device "eth0" priority filter; policy drop;
                ip saddr != @eth_ipv4_address drop
                accept
        }
}
```
```
sudo nft list set netdev filter eth_ipv4_address
table netdev filter {
        set eth_ipv4_address {
                type ipv4_addr
                flags interval
                elements = { 10.0.0.0/24 }
        }
}
```

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