allnet.ts: Restrict request filter to PowerOn

PowerOn uses HTTP slightly incorrectly, but other endpoints use
HTTP incorrectly in different ways (or maybe even correctly, who
knows). Scope the current request interceptor to just PowerOn.

src/allnet.ts

@@ -14,7 +14,7 @@ const app = express();
 // So in the absence of any exotic Transfer-Encoding headers this Content-Type
 // is incorrect and we have to override Express' built-in handling.
 
-app.use(async function(req, res, next) {
+app.use("/sys/servlet/PowerOn", async function(req, res, next) {
   if (req.method !== "POST") {
     return res.status(405).end();
   }