Cygwin: FIFO: add 'record_connection' method

Future commits will have to re-use the code for recording a client
connection.  For convenience, factor out this code into a new method.

winsup/cygwin/fhandler.h

@@ -1279,6 +1279,7 @@ class fhandler_fifo: public fhandler_base
   void delete_client_handler (int);
   bool listen_client ();
   int stop_listen_client ();
+  void record_connection (fifo_client_handler&);
 public:
   fhandler_fifo ();
   bool hit_eof ();