core: firewall integration with DynamicUserNFTSet=

New directive `DynamicUserNFTSet=` provides a method for integrating
configuration of dynamic users into firewall rules with NFT sets.

Example:
```
table inet filter {
        set u {
                typeof meta skuid
        }

        chain service_output {
                meta skuid != @u drop
                accept
        }
}
```

```
/etc/systemd/system/dunft.service
[Service]
DynamicUser=yes
DynamicUserNFTSet=inet:filter:u
ExecStart=/bin/sleep 1000

[Install]
WantedBy=multi-user.target
```

```
$ sudo nft list set inet filter u
table inet filter {
        set u {
                typeof meta skuid
                elements = { 64864 }
        }
}
$ ps -n --format user,group,pid,command -p `pgrep sleep`
    USER    GROUP     PID COMMAND
   64864    64864   55158 /bin/sleep 1000
```

1 files changed, 34 insertions, 0 deletions

diff --git a/man/systemd.exec.xml b/man/systemd.exec.xml
index 50c5c89703..9798a8d999 100644
--- a/man/systemd.exec.xml
+++ b/man/systemd.exec.xml
@@ -3164,6 +3164,40 @@ StandardInputData=V2XigLJyZSBubyBzdHJhbmdlcnMgdG8gbG92ZQpZb3Uga25vdyB0aGUgcnVsZX
   </refsect1>
 
   <refsect1>
+    <title>Firewall Integration</title>
+    <variablelist class='unit-directives'>
+
+      <varlistentry>
+        <term><varname>DynamicUserNFTSet=</varname><replaceable>family</replaceable>:<replaceable>table</replaceable>:<replaceable>set</replaceable></term>
+        <listitem><para>This setting provides a method for integrating <varname>DynamicUser=</varname>
+        configuration into firewall rules with NFT sets. This option expects a whitespace separated list of
+        NFT set definitions. Each definition consists of a colon-separated tuple of NFT address family (one
+        of <literal>arp</literal>, <literal>bridge</literal>, <literal>inet</literal>, <literal>ip</literal>,
+        <literal>ip6</literal>, or <literal>netdev</literal>), table name and set name. The names of tables
+        and sets must conform to lexical restrictions of NFT table names. When the unit starts, the user ID
+        will be appended to the NFT sets and it will be removed when the unit is stopped. Failures to manage
+        the sets will be ignored.</para>
+
+          <para>Example:
+          <programlisting>[Service]
+DynamicUserNFTSet=inet:filter:u</programlisting>
+          Corresponding NFT rules:
+          <programlisting>table inet filter {
+        set u {
+                typeof meta skuid
+        }
+        chain service_output {
+                meta skuid != @u drop
+                accept
+        }
+}</programlisting>
+          </para>
+        </listitem>
+      </varlistentry>
+    </variablelist>
+  </refsect1>
+
+  <refsect1>
     <title>System V Compatibility</title>
     <variablelist class='unit-directives'>