Cygwin: dll_list: stat_real_file_once as dll method

Make stat_real_file_once a method of struct dll, to be more flexible on
where to use.  Also, debug print memory section name queried for a dll.
This is a preparation to query the file id when loading the dll.

winsup/cygwin/dll_init.h

@@ -65,6 +65,7 @@ struct dll
 
   void detach ();
   int init ();
+  bool stat_real_file_once ();
   void nominate_forkable (PCWCHAR);
   bool create_forkable ();
   void run_dtors ()