Stop using kernel proto 42 because it's also used by babeld

l2tpns inserts route into the kernel with proto 42, which is a well-chosen arbitrary value.

Unfortunately, the author of babeld had the same idea. And he has friends in high places so that this (unofficial) assignment is in iproute2. This shows up when displaying routes added by l2tpns on a recent Debian:

$ ip r show dev tun0
XX.XX.XX.XX proto babel scope link

It's only cosmetic, but I think we should change our value to avoid confusion (and also declare the new value in iproute2 to avoid future conflicts)

The problem is that it's a breaking change: external tools such as Bird may match on the proto number to identify routes added by l2tpns. Either we keep the current situation, or we introduce this change in a major release (like 3.0).