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
```
This part of the copyright blurb stems from the GPL use recommendations:
https://www.gnu.org/licenses/gpl-howto.en.html
The concept appears to originate in times where version control was per
file, instead of per tree, and was a way to glue the files together.
Ultimately, we nowadays don't live in that world anymore, and this
information is entirely useless anyway, as people are very welcome to
copy these files into any projects they like, and they shouldn't have to
change bits that are part of our copyright header for that.
hence, let's just get rid of this old cruft, and shorten our codebase a
bit.
Files which are installed as-is (any .service and other unit files, .conf
files, .policy files, etc), are left as is. My assumption is that SPDX
identifiers are not yet that well known, so it's better to retain the
extended header to avoid any doubt.
I also kept any copyright lines. We can probably remove them, but it'd nice to
obtain explicit acks from all involved authors before doing that.
Now we don't support tunnels to be created without a .network file
that is we need a interface index.
This work allows tunnel to be created without a ifindex.
Closes#6695
GRE6 and IP6TNL address should be a IPv6.
fix :
```
Assertion 't->family == AF_INET6' failed at src/network/netdev/tunnel.c:170,
function netdev_ip6gre_fill_message_create(). Aborting.
```
Otherwise we'd fail with an assertion:
Assertion 't->family == AF_INET' failed at ../src/network/netdev/tunnel.c:244, function netdev_vti_fill_message_create(). Aborting.
When assigning addresses, we'd set the family, and later
verify that the address on the other end has the same family.
But when the address was specified as "any", we'd simply unset
the family. Instead, only unset the family if both addresses
are wiped.
Also, don't bother setting family = AF_UNSPEC, since it's the default (0).
