remove incorrect ESRCH error from pthread_kill

posix documents in the rationale and future directions for
pthread_kill that, since the lifetime of the thread id for a joinable
thread lasts until it is joined, ESRCH is not a correct error for
pthread_kill to produce when the target thread has exited but not yet
been joined, and that conforming applications cannot attempt to detect
this state. future versions of the standard may explicitly require
that ESRCH not be returned for this case.

src/thread/pthread_kill.c

@@ -4,7 +4,8 @@ int pthread_kill(pthread_t t, int sig)
 {
 	int r;
 	LOCK(t->killlock);
-	r = t->dead ? ESRCH : -__syscall(SYS_tkill, t->tid, sig);
+	r = t->dead ? (sig+0U >= _NSIG ? EINVAL : 0)
+		: -__syscall(SYS_tkill, t->tid, sig);
 	UNLOCK(t->killlock);
 	return r;
 }