core: add user and group to NFTSet=

core: add user and group to NFTSet=

The benefit of using this setting is that user and group IDs, especially dynamic and random
IDs used by DynamicUser=, can be used in firewall configuration easily.

Example:

```
[Service]
NFTSet=user:inet:filter:serviceuser
```

Corresponding NFT rules:

```
table inet filter {
        set serviceuser {
                typeof meta skuid
        }
        chain service_output {
                meta skuid @serviceuser accept
                drop
        }
}
```

```
$ cat /etc/systemd/system/dunft.service
[Service]
DynamicUser=yes
NFTSet=user:inet:filter:serviceuser
ExecStart=/bin/sleep 1000

[Install]
WantedBy=multi-user.target
$ sudo nft list set inet filter serviceuser
table inet filter {
        set serviceuser {
                typeof meta skuid
                elements = { 64864 }
        }
}
$ ps -n --format user,group,pid,command -p `systemctl show dunft.service -P MainPID`
    USER    GROUP     PID COMMAND
   64864    64864   55158 /bin/sleep 1000
```