* 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/thread.cc

@@ -597,9 +597,9 @@ pthread::cancel ()
     * open ()
     * openat ()
     * pause ()
-      poll ()
+    * poll ()
     * pread ()
-      pselect ()
+    * pselect ()
     * pthread_cond_timedwait ()
     * pthread_cond_wait ()
     * pthread_join ()
@@ -612,7 +612,7 @@ pthread::cancel ()
     * recv ()
     * recvfrom ()
     * recvmsg ()
-      select ()
+    * select ()
     * sem_timedwait ()
     * sem_wait ()
     * send ()