networkd: introduce ipip tunnel

This patch enables basic ipip tunnel support.
It works with kernel module ipip

example conf:

file: ipip.netdev

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

[Tunnel]
Local=192.168.223.238
Remote=192.169.224.239
TTL=64

file: ipip.network

[Match]
Name=em1

[Network]
Tunnel=ipip-tun

[tomegun:
         - drop unused variable
         - take ref when enslaving]
networkd: unit - add cap to load modules

Remember to drop this when the kernel gains autoloading for all netdev kinds.
networkd: manager - don't leak kmod context

Also, keep the kmod_new internal to networkd-manager.c
networkd: Add todo patch kernel for tunnel module alias

Add to todo :

1. Patch kernel to support module alias for tunnel device
2. Remove libkmod dependency from networkd