Lors d'un appel netlink_read, on suppose que NLM_F_MULTI est toujours
vrai, sauf dans le cas d'un appel depuis kernel_callback.

I don't understand why, but it fixes the problems we have, and Gregoire
says it's okay.

This fixes a bug where we would stick to a suboptimal route if the
best route was very stable.

  - 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)

Ajout des parametres newgate et newifindex.

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

neighbour_rxcost() calls update_neighbour which might send some messages,
so make sure we don't call it in the middle of the txcost message.