Skip creating the named pipe on the client if we're using http signaling

* Blocks waiting on the open for write (`cat > signal`), at least for me.

client/snowflake.go

@@ -331,6 +331,7 @@ func main() {
 
 	log.Println("starting")
 
+	if offerURL == "" {
 		// This FIFO receives signaling messages.
 		err = syscall.Mkfifo("signal", 0600)
 		if err != nil {
@@ -344,6 +345,7 @@ func main() {
 		}
 		defer signalFile.Close()
 		go readSignalingMessages(signalFile)
+	}
 
 	webrtc.SetLoggingVerbosity(1)