Cygwin: pty: Fix state mismatch caused in mintty.

- PTY has a bug reported in:
  https://cygwin.com/ml/cygwin/2020-02/msg00067.html.
  This is the result of state mismatch between real pseudo console
  attaching state and state variable. This patch fixes the issue.

winsup/cygwin/fork.cc

@@ -161,6 +161,8 @@ frok::child (volatile char * volatile here)
 	      }
 	  }
       }
+  extern void clear_pcon_attached_to (void); /* fhandler_tty.cc */
+  clear_pcon_attached_to ();
 
   HANDLE& hParent = ch.parent;