demo: don't show meaningless 'odot' arrows for routes

Unexpectedly, and contrary to 'dot', Graphviz does not draw it with penwidth=0
(without, that of the other side is striked). And anyway, we can just look at
the label to see if there's a route.

demo/demo

@@ -279,7 +279,7 @@ def route_svg(ipv4, z = 4, default = type('', (), {'short': None})):
             l.append((min(j, N - j), p, r))
         for j, (l, (p, t), r) in enumerate(sorted(l)):
             l = []
-            arrowhead = 'odot'
+            arrowhead = 'none'
             for r in sorted(r.short for r in r):
                 if r:
                     if r == p.short: