Forget addresses of unreachable nodes.

We clear the cached address used for UDP connections when a node becomes
unreachable. This also prevents host-up scripts from passing the old, cached
address from when the host becomes reachable again from a different address.

src/graph.c

@@ -286,6 +286,9 @@ void sssp_bfs(void) {
 				free(envp[i]);
 
 			subnet_update(n, NULL, n->status.reachable);
+
+			if(!n->status.reachable)
+				update_node_udp(n, NULL);
 		}
 	}
 }