networkd: introduce sit tunnel

This patch introduces sit tunnel support to networkd

Example conf:
file: sit.netdev

[NetDev]
Name=sit-tun
Kind=sit
MTUBytes=1480

[Tunnel]
Local=10.65.223.238
Remote=10.65.223.239

file: sit.network

[Match]
Name=em1

[Network]
Tunnel=sit-tun

[tomegun: rebased]
TODO

IFLA_IPTUN_PMTUDISC
IFLA_IPTUN_FLAGS (ISA_TAP)

FLA_IPTUN_6RD_PREFIX
FLA_IPTUN_6RD_RELAY_PREFIX
IFLA_IPTUN_6RD_PREFIXLEN
IFLA_IPTUN_6RD_RELAY_PREFIXLEN