Cygwin: pty: Add FreeConsole to destructor of pty slave.

- When pseudo console is closed, all the processes attched to the
  pseudo console are terminated. This causes the problem reported
  in https://sourceware.org/pipermail/cygwin/2020-March/244046.html.
  This patch fixes the issue.

winsup/cygwin/fhandler_tty.cc

@@ -706,8 +706,15 @@ fhandler_pty_slave::fhandler_pty_slave (int unit)
 fhandler_pty_slave::~fhandler_pty_slave ()
 {
   if (!get_ttyp ())
-    /* Why comes here? Who clears _tc? */
-    return;
+    {
+      /* Why comes here? Who clears _tc? */
+      if (freeconsole_on_close)
+	{
+	  FreeConsole ();
+	  pcon_attached_to = -1;
+	}
+      return;
+    }
   if (get_pseudo_console ())
     {
       int used = 0;