fix detection of host netns when weave script runs inside container

$$ refers to the pid in the container's process namespace, so using it
as a reference into /hostproc is entirely wrong.

Fixes #1162.

weave

@@ -359,7 +359,7 @@ netnsenter() {
 }
 
 connect_container_to_bridge() {
-    if [ -h "$PROCFS/$CONTAINER_PID/ns/net" -a -h "$PROCFS/$$/ns/net" -a "$(readlink $PROCFS/$CONTAINER_PID/ns/net)" = "$(readlink $PROCFS/$$/ns/net)" ] ; then
+    if [ -h "$PROCFS/$CONTAINER_PID/ns/net" -a -h "/proc/$$/ns/net" -a "$(readlink $PROCFS/$CONTAINER_PID/ns/net)" = "$(readlink /proc/$$/ns/net)" ] ; then
         echo "Container is running in the host network namespace, and therefore cannot be" >&2
         echo "connected to weave. Perhaps the container was started with --net=host." >&2
         return 1