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/internal/libc.h

@@ -18,6 +18,7 @@ struct __libc {
 	FILE *ofl_head;
 	int ofl_lock;
 	void (*lockfile)(FILE *);
+	void (*sigtimer)();
 };