networkd: introduce vxcan netdev. (#7150)

Similar to the virtual ethernet driver veth, vxcan implements a
local CAN traffic tunnel between two virtual CAN network devices.
When creating a vxcan, two vxcan devices are created as pair
When one end receives the packet it appears on its pair and vice
versa. The vxcan can be used for cross namespace communication.

man/systemd.netdev.xml

@@ -180,6 +180,10 @@
           <row><entry><varname>vcan</varname></entry>
           <entry>The virtual CAN driver (vcan). Similar to the network loopback devices, vcan offers a virtual local CAN interface.</entry></row>
 
+          <row><entry><varname>vxcan</varname></entry>
+          <entry>The virtual CAN tunnel driver (vxcan). Similar to the virtual ethernet driver veth, vxcan implements a local CAN traffic tunnel between two virtual CAN network devices. When creating a vxcan, two vxcan devices are created as pair. When one end receives the packet it appears on its pair and vice versa. The vxcan can be used for cross namespace communication.
+          </entry></row>
+
         </tbody>
       </tgroup>
     </table>
@@ -894,6 +898,22 @@
         </varlistentry>
       </variablelist>
   </refsect1>
+  <refsect1>
+  <title>[VXCAN] Section Options</title>
+      <para>The <literal>[VXCAN]</literal> section only applies for
+      netdevs of kind <literal>vxcan</literal> and accepts the
+      following key:</para>
+
+      <variablelist class='network-directives'>
+        <varlistentry>
+          <term><varname>Peer=</varname></term>
+          <listitem>
+            <para>The peer interface name used when creating the netdev.
+            This option is compulsory.</para>
+          </listitem>
+        </varlistentry>
+      </variablelist>
+  </refsect1>
   <refsect1>
     <title>[Tun] Section Options</title>