"waiting for data" msg should appear immediately on F command on a fifo.

On a file, poll() returns POLLIN (readable data) even if we are at
end of file. This allows the code in ch_get to see that there is no
data available and display the "waiting for data" message. But on a
fifo, if there is no data available, poll() will block until the
timeout, so at eof on a pipe we will be stuck in the poll and never
display the waiting message. This commit changes the timeout so that
on the first read on a fifo (when there is no waiting message displayed)
we will use timeout=0 and return immediately. This allows ch_get to
display the waiting message. After the message is displayed,
subsequent reads will use an infinite timeout.

Related to #555.

os.c

@@ -119,7 +119,7 @@ public void init_poll(void)
 static int check_poll(int fd, int tty)
 {
 	struct pollfd poller[2] = { { fd, POLLIN, 0 }, { tty, POLLIN, 0 } };
-	int timeout = (waiting_for_data && !(scanning_eof && follow_mode == FOLLOW_NAME)) ? -1 : waiting_for_data_delay;
+	int timeout = (waiting_for_data && !(scanning_eof && follow_mode == FOLLOW_NAME)) ? -1 : (ignore_eoi && !waiting_for_data) ? 0 : waiting_for_data_delay;
 	if (!any_data)
 	{
 		/*