* select.cc (cygwin_select): Make degenerate case cancelable.

(select_stuff::destroy): New inline method to delete memory taken
	by select_stuff.
	(select_stuff::~select_stuff): Call destroy.
	(select_stuff::wait): Add case to allow canceling select.
	* select.h (select_stuff::destroy): Declare.
	* thread.cc: Mark poll, pselect and poll as cancelable.

winsup/cygwin/select.h

@@ -83,6 +83,7 @@ public:
   int poll (fd_set *readfds, fd_set *writefds, fd_set *exceptfds);
   int wait (fd_set *readfds, fd_set *writefds, fd_set *exceptfds, DWORD ms);
   void cleanup ();
+  void destroy ();
   select_stuff (): always_ready (0), windows_used (0), start (0),
 		   device_specific_pipe (0),
 		   device_specific_socket (0),