input_curl: use select() to eliminate busy loop during connect

When decoder_run_song() (decoder_thread.c) waits for the input stream
to become ready, it did that in a busy loop.  Add a select() call to
input_curl_buffer() during connect/handshake (i.e. before the first
chunk of body data was received), to let the CPU relax.

src/input_curl.c

@@ -470,6 +470,12 @@ input_curl_buffer(struct input_stream *is)
 
 	c->buffered = false;
 
+	if (!is->ready)
+		/* not ready yet means the caller is waiting in a busy
+		   loop; relax that by calling select() on the
+		   socket */
+		input_curl_select(c);
+
 	do {
 		mcode = curl_multi_perform(c->multi, &running_handles);
 	} while (mcode == CURLM_CALL_MULTI_PERFORM &&