watcher: Check for cancellation if poll() fails with EINTR

With LinuxThreads, poll() is unfortunately no cancellation point. It seems
that poll gets woken up after cancellation, but we actively must check
for cancellation before re-entering poll to properly shut down the watcher
thread.

src/libstrongswan/processing/watcher.c

@@ -345,6 +345,13 @@ static job_requeue_t watch(private_watcher_t *this)
 		old = thread_cancelability(TRUE);
 
 		res = poll(pfd, count, -1);
+		if (res == -1 && errno == EINTR)
+		{
+			/* LinuxThreads interrupts poll(), but does not make it a
+			 * cancellation point. Manually test if we got cancelled. */
+			thread_cancellation_point();
+		}
+
 		thread_cancelability(old);
 		thread_cleanup_pop(FALSE);