networkd: Introduce ip6gre and ip6gretap

networkd: Introduce ip6gre and ip6gretap

This patch introduces ipv6 gre and gretap.

test:

ip6gre.netdev:
[NetDev]
Name=ip6gretap
Kind=ip6gretap

[Tunnel]
Local=2a00:ffde:4567:edde::4987
Remote=2001:473:fece:cafe::5179

ip6gre.network:
[Match]
Name=eno16777736

[Network]
Tunnel=ip6gretap

ip link

6: ip6gre@eno16777736: <POINTOPOINT,NOARP> mtu 1448 qdisc noop state
DOWN mode DEFAULT group default
    link/gre6 2a:00:ff:de:45:67:ed:de:00:00:00:00:00:00:49:87 peer
20:01:04:73:fe:ce:ca:fe:00:00:00:00:00:00:51:79