unix: don't flush tty on switch to raw mode

Drain, don't flush the tty when switching from cooked to raw mode. Prevents
buffered keystrokes from getting lost. Switching back to cooked mode still
flushes.

Fixes joyent/node#2744.

src/unix/tty.c

@@ -76,8 +76,8 @@ int uv_tty_set_mode(uv_tty_t* tty, int mode) {
     raw.c_cc[VMIN] = 1;
     raw.c_cc[VTIME] = 0;
 
-    /* Put terminal in raw mode after flushing */
-    if (tcsetattr(fd, TCSAFLUSH, &raw)) {
+    /* Put terminal in raw mode after draining */
+    if (tcsetattr(fd, TCSADRAIN, &raw)) {
       goto fatal;
     }