Use TCSANOW to prevent from discarding the input buffer

TCSAFLUSH discards the buffer read before the mode change, which makes
IRB ignore the buffer input immediately after invoked.  TCSANOW
preserves the buffer.

ext/io/console/console.c

@@ -23,7 +23,7 @@ typedef struct termios conmode;
 static int
 setattr(int fd, conmode *t)
 {
-    while (tcsetattr(fd, TCSAFLUSH, t)) {
+    while (tcsetattr(fd, TCSANOW, t)) {
 	if (errno != EINTR) return 0;
     }
     return 1;