Cygwin: pty: Use pinfo() rather than kill() with signal 0.

- PTY code has a problem that tcsh is terminated if the following
  command is executed.
    true; chcp &
  This seems to be caused by invalid pointer access which occurs
  when the process exits during the kill() code is execuetd. This
  patch avoids the issue by not using kill().

winsup/cygwin/fhandler.h

@@ -2211,7 +2211,7 @@ class fhandler_pty_slave: public fhandler_pty_common
   {
     if (!mask && get_ttyp ()->pcon_pid &&
 	get_ttyp ()->pcon_pid != myself->pid &&
-	kill (get_ttyp ()->pcon_pid, 0) == 0)
+	!!pinfo (get_ttyp ()->pcon_pid))
       return;
     get_ttyp ()->mask_switch_to_pcon_in = mask;
   }