This is mainly to see what's necessary to create them (in case we
integrate this into the daemon) and to experiment in our testing
environment without having to add a patched version of iproute2 (the
4.20.0 version in stretch-backports doesn't support XFRM interfaces
yet). The regular version of iproute2 can be used for other operations
with these interfaces (delete, up, addrs etc.).
Tobias Brunner
888593bb5b