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

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
```