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 }
        }
}
```

src/basic/parse-util.h

@@ -146,3 +146,5 @@ int parse_oom_score_adjust(const char *s, int *ret);
  * to a loadavg_t. */
 int store_loadavg_fixed_point(unsigned long i, unsigned long f, loadavg_t *ret);
 int parse_loadavg_fixed_point(const char *s, loadavg_t *ret);
+
+bool nft_identifier_bad(const char *id);