major improvements to cancellation handling

- there is no longer any risk of spoofing cancellation requests, since
  the cancel flag is set in pthread_cancel rather than in the signal
  handler.

- cancellation signal is no longer unblocked when running the
  cancellation handlers. instead, pthread_create will cause any new
  threads created from a cancellation handler to unblock their own
  cancellation signal.

- various tweaks in preparation for POSIX timer support.

src/thread/pthread_cancel.c

@@ -2,5 +2,6 @@
 
 int pthread_cancel(pthread_t t)
 {
+	t->cancel = 1;
 	return pthread_kill(t, SIGCANCEL);
 }