It doesn't appear to be useful, and complicates uselessly complicates matters.

Significantly simplifies the xroute logic.

We now remember the number of kernel routes from one call of check_xroutes
to the next to avoid calling down into the kernel multiple times.

This prevents a stale route from remaining installed when we get a new xroute.

It was difficult to write, there's no reason it should easy to read.

This is safe, since no part of the code ever holds a pointer to an xroute.

This ensures that retractions are sent too.

Also removes stale local routes.

On linux, listen to address and link change events using the netlink socket,
and react to them by dumping all local addresses or all local interfaces,
respectively, to keep babel's view up to date.
This breaks MacOS, as kernel_socket.c needs to grow a kernel_addresses()
function.

Xroutes are now generated dynamically as needed.

  - Fusion de kernel_listen dans kernel_read.
  - Fusion de monitor_kernel_routes dans filter_kernel_routes.
  - En cas d'erreur d'entree/sortie on ferme les sockets.
    - nl_command est rouverte au besoin
    - nl_listen doit etre rouverte a l'aide de kernel_setup_socket(1)

This is not compatible with version 0.  Uses 24-byte messages and
doesn't distinguish routes from xroutes.