cmd/strelaysrv, cmd/strelaypoolsrv: Sanitize query strings (fixes #8314) (#8315)

Use the proper encoding function in the relay server when constructing
the URL. In the pool server, parse and re-encode the query values to
sanitize whatever the client sent.

cmd/strelaypoolsrv/main.go

@@ -368,6 +368,11 @@ func handlePostRequest(w http.ResponseWriter, r *http.Request) {
 		return
 	}
 
+	// Canonicalize the URL. In particular, parse and re-encode the query
+	// string so that it's guaranteed to be valid.
+	uri.RawQuery = uri.Query().Encode()
+	newRelay.URL = uri.String()
+
 	if relayCert != nil {
 		advertisedId := uri.Query().Get("id")
 		idFromCert := protocol.NewDeviceID(relayCert.Raw).String()