Cygwin: pty: Revise code waiting for forwarding by master_fwd_thread.

- Though this rarely happens, sometimes the first printing of non-
  cygwin process does not displayed correctly. To fix this issue,
  the code for waiting for forwarding by master_fwd_thread is revised.

winsup/cygwin/tty.h

@@ -106,6 +106,7 @@ private:
   int num_pcon_attached_slaves;
   UINT term_code_page;
   bool need_redraw_screen;
+  HANDLE fwd_done;
 
 public:
   HANDLE from_master () const { return _from_master; }