This allows 0 to appear within a channel list, which is consistent with
draft-chroboczek-babel-diversity-routing-01.

Two main modifications:
  - source-specific routes are putted in front of the RIB,
  - route_stream now allows traversing all routes, all installed routes,
    or source-specific installed routes.

Was used in the first version of the disambiguation.c algorithm.

This reverts commit 9edf0a14.

Thanks to Pierre Pfister.

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.

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

This avoids the quadratic behaviour of expire_sources.

This changes the route table to be a sorted table of linked lists of
routes to a given prefix, which makes most route operations behave in
O(log n).  Insertion and flushing of a prefix is still O(n), but these
are fairly rare operations.

A nice side-effect is that the route table is now private to route.c,
which should make it easy to switch to a different data structure in
the future.

This moves all metric computation back to route.[ch],
where it belongs.

This is useful in order to provide extra information to a graphic
front-end.  In can cause the RIB to explode in some pathological cases,
so it is not the default.

Working out exactly when to call local_notify_neighbour is too
error-prone.  Call it systematically, at the cost of some extra
local traffic.

Reported by Gabriel Kerneis.

This avoids desynchronisation between Babel and the kernel.

This avoids searching for the source again.