Cygwin: pty: Use system NLS function instead of PTY's own one.

- Since calling system __loadlocale() caused execution error,
  PTY used its own NLS function. The cause of the error has been
  found, the corresponding code has been rewritten using system
  function.

winsup/cygwin/tty.cc

@@ -243,7 +243,7 @@ tty::init ()
   mask_switch_to_pcon_in = false;
   pcon_pid = 0;
   num_pcon_attached_slaves = 0;
-  TermCodePage = 20127; /* ASCII */
+  term_code_page = 0;
   need_clear_screen = false;
 }