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 }
        }
}
```

test/fuzz/fuzz-network-parser/directives

@@ -132,6 +132,7 @@ RouteMTUBytes=
 FallbackLeaseLifetimeSec=
 Use6RD=
 NetLabel=
+NFTSet=
 [DHCPv6]
 UseAddress=
 UseDelegatedPrefix=
@@ -154,6 +155,7 @@ IAID=
 DUIDType=
 DUIDRawData=
 NetLabel=
+NFTSet=
 [DHCPv6PrefixDelegation]
 SubnetId=
 Announce=
@@ -171,6 +173,7 @@ ManageTemporaryAddress=
 Token=
 RouteMetric=
 NetLabel=
+NFTSet=
 [Route]
 Destination=
 Protocol=
@@ -257,6 +260,8 @@ DHCPv6PrefixDelegation=
 DHCPPrefixDelegation=
 BatmanAdvanced=
 IPoIB=
+IPv4NFTSet=
+IPv6NFTSet=
 [IPv6Prefix]
 Prefix=
 OnLink=
@@ -348,6 +353,7 @@ Managed=
 OtherInformation=
 UplinkInterface=
 NetLabel=
+NFTSet=
 [IPv6PrefixDelegation]
 RouterPreference=
 DNSLifetimeSec=