This avoids overflow by expanding the range.

Thanks to Yoann Canal.

No hello is -1, not 0.

IHUs are now sent synchronously with hellos.  There's no longer an explicit
IHU interval, we send IHUs for all neighbours every three hellos, and for
marginal neighbours with each hello.

Instead of neigh->id[0] = 0xFF.

This is more taken care of in update_neighbour_metric.

Gregoire and Julien have been reporting that links with up to 90%
hello loss can be useful in extreme circumstances.

This change prevents Babel from summarily discarding such very marginal
links.  This should not have any effect when better links are available.