Remove limiting of reachability information to 5 hellos.

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.

neighbour.c

@@ -262,7 +262,7 @@ neighbour_rxcost(struct neighbour *neigh)
 
     delay = timeval_minus_msec(&now, &neigh->hello_time);
 
-    if((reach & 0xF800) == 0 || delay >= 180000) {
+    if((reach & 0xFFF0) == 0 || delay >= 180000) {
         return INFINITY;
     } else if(neigh->network->wired) {
         /* To lose one hello is a misfortune, to lose two is carelessness. */