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.

It is no longer used.

This now happens in network_up.

This now happens automatically.

This is now handled indirectly through neighbour_cost.

We call start_message directly there.

This avoids a spurious warning.

This avoids a crash if we attempt to compute jitter intervals before
the interface is officially up.