[Babel-users] Fix for infinite txcost

[Juliusz Chroboczek]

I've hopefully just fixed an annoying race condition in babeld.

The symptoms were that all neighbours over an interface would have
infinite txcost (and hence be unreachable) until babeld was restarted.
The issue was that babeld would notice that the interface was up
before a link-local address was assigned; this should normally be
handled by failing to join the Babel multicast group, which was
handled correctly, but for some reason it sometimes wasn't.

The fix is in the master branch, together with a bunch of cleanups.
I'll make a less invasive fix for the 1.3 branch.

-- Juliusz