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

	* 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.