reinstate creation of node.url files upon startup

a recent purge of the start.html code also took away the logic that wrote
'node.url' into the node root.  this is required for the tahoe cli tool to 
find the node.  this puts back a limited fraction of that code, so that the
node writes out a node.url file upon startup.

src/allmydata/client.py

@@ -97,7 +97,8 @@ class Client(node.Node, Referenceable, testutil.PollMixin):
         self.log("init_web(webport=%s)", args=(webport,))
 
         from allmydata.webish import WebishServer
-        ws = WebishServer(webport)
+        nodeurl_path = os.path.join(self.basedir, "node.url")
+        ws = WebishServer(webport, nodeurl_path)
         if self.get_config("webport_allow_localfile") is not None:
             ws.allow_local_access(True)
         self.add_service(ws)