We already have IP4LL and maybe should re-use that also for IPv6.
However, when adding the prefix route for IPv6 link local addresses,
we want to add it with protocol "kernel", unlike "user" for IPv4.
There is no strong reason for this. I don't think the protocol matters
much. But up to now kernel automatically adds this prefix route, so
as we are going to change that and let NetworkManager add it, keep the
protocol at "kernel".