Cygwin: FIFO: fix and simplify listen_client_thread

Remove fifo_client_handler::connect and move its code into
listen_client_thread.  That way we can check the return status when a
client handler's connect_evt is signaled.  Previously we incorrectly
assumed there was a successful connection.

Also simplify listen_client_thread in the following ways:

- Replace fhandler_fifo::disconnect_and_reconnect by a new
  delete_client_handler method.  Now we just delete invalid client
  handlers rather than trying to re-use them.

- Try to maintain a client handler list that consists of connected
  client handlers and exactly one that is listening for a connection.
  This allows us to call WaitForMultipleObjects with only two wait
  objects.

- Remove 'dummy_evt' from the fifo_client_handler struct; it is no
  longer needed.

- On exit from listen_client_thread, delete the "extra" (listening)
  client handler.  Otherwise there could be a connection that doesn't
  get recorded in the client handler list.  This could happen when a
  file descriptor is being duplicated.

winsup/cygwin/fhandler.h

@@ -1250,13 +1250,10 @@ struct fifo_client_handler
   fhandler_base *fh;
   fifo_client_connect_state state;
   HANDLE connect_evt;
-  HANDLE dummy_evt;		/* Never signaled. */
-  fifo_client_handler () : fh (NULL), state (fc_unknown), connect_evt (NULL),
-			   dummy_evt (NULL) {}
+  fifo_client_handler () : fh (NULL), state (fc_unknown), connect_evt (NULL) {}
   fifo_client_handler (fhandler_base *_fh, fifo_client_connect_state _state,
-		       HANDLE _connect_evt, HANDLE _dummy_evt)
-    : fh (_fh), state (_state), connect_evt (_connect_evt),
-      dummy_evt (_dummy_evt) {}
+		       HANDLE _connect_evt)
+    : fh (_fh), state (_state), connect_evt (_connect_evt) {}
   int connect ();
   int close ();
 };
@@ -1278,8 +1275,8 @@ class fhandler_fifo: public fhandler_base
   NTSTATUS npfs_handle (HANDLE &);
   HANDLE create_pipe_instance (bool);
   NTSTATUS open_pipe ();
-  int disconnect_and_reconnect (int);
   int add_client_handler ();
+  void delete_client_handler (int);
   bool listen_client ();
   int stop_listen_client ();
 public: