Cygwin: pty: Clear discard_input flag on master write()

Currently, the first transfer_input() after Ctrl-C does not work
because discard_input flag remains asserted. This can cause loosing
typeahead input for non-cygwin app after Ctrl-C. With this patch,
the discard_input flag is cleared on master write() because the
input is new valid input after discarding input.

Fixes: 4e16e57 ("Cygwin: pty: Discard input already accepted on interrupt.")
Signed-off-by: Takashi Yano <takashi.yano@nifty.ne.jp>
Reviewed-by: Corinna Vinschen <corinna@vinschen.de>

Author: tyan0

winsup/cygwin/fhandler/pty.cc

@@ -2149,6 +2149,8 @@ fhandler_pty_master::write (const void *ptr, size_t len)
 
   push_process_state process_state (PID_TTYOU);
 
+  get_ttyp ()->discard_input = false;
+
   if (get_ttyp ()->pcon_start)
     { /* Reaches here when pseudo console initialization is on going. */
       /* Pseudo condole support uses "CSI6n" to get cursor position.