core: firewall integration of cgroups with NFTSet=

core: firewall integration of cgroups with NFTSet=

New directive `NFTSet=` provides a method for integrating dynamic cgroup IDs
into firewall rules with NFT sets. The benefit of using this setting is to be
able to use control group as a selector in firewall rules easily and this in
turn allows more fine grained filtering. Also, NFT rules for cgroup matching
use numeric cgroup IDs, which change every time a service is restarted, making
them hard to use in systemd environment.

This option expects a whitespace separated list of NFT set definitions. Each
definition consists of a colon-separated tuple of source type (only "cgroup"),
NFT address family (one of "arp", "bridge", "inet", "ip", "ip6", or "netdev"),
table name and set name. The names of tables and sets must conform to lexical
restrictions of NFT table names. The type of the element used in the NFT filter
must be "cgroupsv2". When a control group for a unit is realized, the cgroup ID
will be appended to the NFT sets and it will be be removed when the control
group is removed.  systemd only inserts elements to (or removes from) the sets,
so the related NFT rules, tables and sets must be prepared elsewhere in
advance.  Failures to manage the sets will be ignored.

If the firewall rules are reinstalled so that the contents of NFT sets are
destroyed, command systemctl daemon-reload can be used to refill the 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]
NFTSet=cgroup:inet:filter:timesyncd
```

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