Reported by Antoine Beaupré.

This is a systematic way of implementing hyseresis in route
selection, and should solve the issues we're seeing with ETX
jitter and feedback.  The default half-time is 4s, which might
be too small.

This fixes an issue where find_best_route could very occasionally
return a route that was not feasible.

This is needed for the stability aware code.

This is brittle, and prevents me from making route selection smarter.

This is the functionality described in Appendix C of RFC 6126.  Its
main purpose is to avoid keeping a full source table, which makes it
possible to implement a subset of Babel in just a few hundred lines of
code.  However, the code for maintaining the source table is already
there, and a parasitic implementation can be simulated using filtering
-- so it makes little sense to keep the functionality.

Thanks to Denis Ovsienko.

FreeBSD system headers have their own "struct route", which made it
impossible to compile babeld. Switching babeld to "struct babel_route".

We now limit resends to just 3 copies, both for requests and updates,
since there are enough mechanisms in the protocol to handle packet
loss.

They are no longer used except for configuring interfaces with no
explicit configuration.

This feature was little used, and complicated the code somewhat.

Nodes send wildcard requests at boot, which allows them to acquire
a full routing table in a timely manner.  Unfortunately, this causes
an update storm when a large network boots.

We used to allocate enough buffer space to send one full-size frame.
Unfortunately, in large networks this causes our update coalescing
strategy to fail, just where it is the most useful.  Allocate enough
buffers to fit a full update.