* fhandler.h class fhandler_socket): Declare new method

`set_close_on_exec'.
        * fhandler_socket.cc (fhandler_socket::set_close_on_exec):
        New method.

winsup/cygwin/fhandler.h

@@ -383,6 +383,7 @@ public:
   void hclose (HANDLE) {close ();}
   int dup (fhandler_base *child);
 
+  void set_close_on_exec (int val);
   virtual void fixup_before_fork_exec (DWORD);
   void fixup_after_fork (HANDLE);
   void fixup_after_exec (HANDLE parent) { fixup_after_fork (parent); }