Fix master socket.io broken by a3fc1dd

Add missing attach calls to the master socket.io object that was
accidentally removed by the refactoring in a3fc1dd.  Unfortunately I've
been unable to determine if socket.io actually supports listening on
multiple servers at the same time, this needs further investigation.

master.js

@@ -945,6 +945,7 @@ async function startServer() {
 	if (httpPort) {
 		let httpServer = require("http").Server(app);
 		await listen(httpServer, httpPort);
+		io.attach(httpServer);
 		console.log("Listening for HTTP on port %s...", httpServer.address().port);
 	}
 
@@ -965,6 +966,10 @@ async function startServer() {
 			cert: certificate
 		}, app)
 		await listen(httpsServer, httpsPort);
+
+		// XXX I'm uncertain whether or not socket.io actually supports
+		// attaching to multiple servers at the same time.
+		io.attach(httpsServer);
 		console.log("Listening for HTTPS on port %s...", httpsServer.address().port);
 	}
 }