networkd and sd-netlink: add support for Generic netlink And FooOverUDP to IPIP tunnel

This work add support to generic netlink to sd-netlink.
See https://lwn.net/Articles/208755/

networkd: add support FooOverUDP support to IPIP tunnel netdev
https://lwn.net/Articles/614348/

Example conf:

/lib/systemd/network/1-fou-tunnel.netdev
```
[NetDev]
Name=fou-tun
Kind=fou

[FooOverUDP]
Port=5555
Protocol=4

```

/lib/systemd/network/ipip-tunnel.netdev
```
[NetDev]
Name=ipip-tun
Kind=ipip

[Tunnel]
Independent=true
Local=10.65.208.212
Remote=10.65.208.211
FooOverUDP=true
FOUDestinationPort=5555
```

$ ip -d link show ipip-tun
```
5: ipip-tun@NONE: <POINTOPOINT,NOARP> mtu 1472 qdisc noop state DOWN mode DEFAULT group default qlen 1000
    link/ipip 10.65.208.212 peer 10.65.208.211 promiscuity 0
    ipip remote 10.65.208.211 local 10.65.208.212 ttl inherit pmtudisc encap fou encap-sport auto encap-dport 5555 noencap-csum noencap-csum6 noencap-remcsum numtxqueues 1 numrxqueues 1 gso_max_size 65536 gso_max_segs 65535
```

1 files changed, 98 insertions, 0 deletions

diff --git a/man/systemd.netdev.xml b/man/systemd.netdev.xml
index d950eb578b..380d0088ce 100644
--- a/man/systemd.netdev.xml
+++ b/man/systemd.netdev.xml
@@ -163,6 +163,10 @@
 
           <row><entry><varname>netdevsim</varname></entry>
           <entry> A simulator. This simulated networking device is used for testing various networking APIs and at this time is particularly focused on testing hardware offloading related interfaces.</entry></row>
+
+          <row><entry><varname>fou</varname></entry>
+          <entry>Foo-over-UDP tunneling.</entry></row>
+
         </tbody>
       </tgroup>
     </table>
@@ -879,8 +883,76 @@
           </para>
         </listitem>
       </varlistentry>
+      <varlistentry>
+        <term><varname>FooOverUDP=</varname></term>
+        <listitem>
+          <para>A boolean. Specifies whether <varname>FooOverUDP=</varname> tunnel is to be configured.
+          Defaults to false. For more detail information see
+          <ulink url="https://lwn.net/Articles/614348">Foo over UDP</ulink></para>
+        </listitem>
+      </varlistentry>
+      <varlistentry>
+        <term><varname>FOUDestinationPort=</varname></term>
+        <listitem>
+          <para>The <varname>FOUDestinationPort=</varname> specifies the UDP destination port for encapsulation.
+          This field is mandatory and is not set by default.</para>
+        </listitem>
+      </varlistentry>
+      <varlistentry>
+        <term><varname>FOUSourcePort=</varname></term>
+        <listitem>
+          <para>The <constant>FOUSourcePort=</constant> specifies the UDP source port for encapsulation. Defaults to <varname>0</varname>,
+          that is, the source port for packets is left to the network stack to decide.</para>
+        </listitem>
+      </varlistentry>
+      <varlistentry>
+        <term><varname>Encapsulation=</varname></term>
+        <listitem>
+          <para>Accepts the same key as <literal>[FooOverUDP]</literal></para>
+        </listitem>
+      </varlistentry>
     </variablelist>
   </refsect1>
+
+  <refsect1>
+    <title>[FooOverUDP] Section Options</title>
+
+    <para>The <literal>[FooOverUDP]</literal> section only applies for
+    netdevs of kind <literal>fou</literal> and accepts the
+    following keys:</para>
+
+    <variablelist class='network-directives'>
+      <varlistentry>
+        <term><varname>Protocol=</varname></term>
+        <listitem>
+          <para>The <varname>Protocol=</varname> specifies the protocol number of the
+          packets arriving at the UDP port. This field is mandatory and is not set by default. Valid range is 1-255.</para>
+        </listitem>
+      </varlistentry>
+      <varlistentry>
+        <term><varname>Encapsulation=</varname></term>
+        <listitem>
+          <para>Specifies the encapsulation mechanism used to store networking packets of various protocols inside the UDP packets. Supports the following values:
+
+          <literal>FooOverUDP</literal> provides the simplest no frills model of UDP encapsulation, it simply encapsulates
+          packets directly in the UDP payload.
+          <literal>GenericUDPEncapsulation</literal> is a generic and extensible encapsulation, it allows encapsulation of packets for any IP
+          protocol and optional data as part of the encapsulation.
+          For more detailed information see <ulink url="https://lwn.net/Articles/615044">Generic UDP Encapsulation</ulink>.
+          Defaults to <literal>FooOverUDP</literal>.
+          </para>
+        </listitem>
+      </varlistentry>
+      <varlistentry>
+        <term><varname>Port=</varname></term>
+        <listitem>
+          <para>Specifies the port number, where the IP encapsulation packets will arrive. Please take note that the packets
+          will arrive with the encapsulation will be removed. Then they will be manually fed back into the network stack, and sent ahead
+          for delivery to the real destination. This option is mandatory.</para>
+        </listitem>
+        </varlistentry>
+      </variablelist>
+  </refsect1>
   <refsect1>
     <title>[Peer] Section Options</title>
 
@@ -1380,6 +1452,32 @@ Remote=192.169.224.239
 TTL=64</programlisting>
     </example>
     <example>
+      <title>/etc/systemd/network/1-fou-tunnel.netdev</title>
+      <programlisting>[NetDev]
+Name=fou-tun
+Kind=fou
+
+[FooOverUDP]
+Port=5555
+Protocol=4
+      </programlisting>
+    </example>
+    <example>
+      <title>/etc/systemd/network/25-fou-ipip.netdev</title>
+      <programlisting>[NetDev]
+[NetDev]
+Name=ipip-tun
+Kind=ipip
+
+[Tunnel]
+Independent=true
+Local=10.65.208.212
+Remote=10.65.208.211
+FooOverUDP=true
+FOUDestinationPort=5555
+      </programlisting>
+    </example>
+    <example>
       <title>/etc/systemd/network/25-tap.netdev</title>
       <programlisting>[NetDev]
 Name=tap-test