fix async-cancel-safety of pthread_cancel

the previous commit addressing async-signal-safety issues around
pthread_kill did not fully fix pthread_cancel, which is also required
(albeit rather irrationally) to be async-cancel-safe.

without blocking implementation-internal signals, it's possible that,
when async cancellation is enabled, a cancel signal sent by another
thread interrupts pthread_kill while the killlock for a targeted
thread is held. as a result, the calling thread will terminate due to
cancellation without ever unlocking the targeted thread's killlock,
and thus the targeted thread will be unable to exit.

src/thread/pthread_kill.c

@@ -5,7 +5,10 @@ int pthread_kill(pthread_t t, int sig)
 {
 	int r;
 	sigset_t set;
-	__block_app_sigs(&set);
+	/* Block not just app signals, but internal ones too, since
+	 * pthread_kill is used to implement pthread_cancel, which
+	 * must be async-cancel-safe. */
+	__block_all_sigs(&set);
 	LOCK(t->killlock);
 	r = t->tid ? -__syscall(SYS_tkill, t->tid, sig)
 		: (sig+0U >= _NSIG ? EINVAL : 0);