Add configuration to disable IPv6, since old method no longer works.

fixes #108

util/sysctl_addon

@@ -15,3 +15,8 @@ net.ipv4.neigh.default.gc_thresh3 = 16384
 
 # Mininet: increase routing table size
 net.ipv4.route.max_size=32768
+
+# Mininet: disable IPv6
+net.ipv6.conf.all.disable_ipv6 = 1
+net.ipv6.conf.default.disable_ipv6 = 1
+net.ipv6.conf.lo.disable_ipv6 = 1