core: firewall integration with ControlGroupNFTSet=

core: firewall integration with ControlGroupNFTSet=

New directive `ControlGroupNFTSet=` provides a method for integrating services
into firewall rules with NFT sets.

Example:

```
table inet filter {
...
        set timesyncd {
                type cgroupsv2
        }

        chain ntp_output {
                socket cgroupv2 != @timesyncd counter drop
                accept
        }
...
}
```

/etc/systemd/system/systemd-timesyncd.service.d/override.conf
```
[Service]
ControlGroupNFTSet=inet:filter:timesyncd
```

```
$ sudo nft list set inet filter timesyncd
table inet filter {
        set timesyncd {
                type cgroupsv2
                elements = { "system.slice/systemd-timesyncd.service" }
        }
}
```