fix race condition in pthread_kill

if thread id was reused by the kernel between the time pthread_kill
read it from the userspace pthread_t object and the time of the tgkill
syscall, a signal could be sent to the wrong thread. the tgkill
syscall was supposed to prevent this race (versus the old tkill
syscall) but it can't; it can only help in the case where the tid is
reused in a different process, but not when the tid is reused in the
same process.

the only solution i can see is an extra lock to prevent threads from
exiting while another thread is trying to pthread_kill them. it should
be very very cheap in the non-contended case.

src/thread/pthread_kill.c

@@ -2,5 +2,9 @@
 
 int pthread_kill(pthread_t t, int sig)
 {
-	return -__syscall(SYS_tgkill, t->pid, t->tid, sig);
+	int r;
+	__lock(&t->killlock);
+	r = t->dead ? ESRCH : -__syscall(SYS_tgkill, t->pid, t->tid, sig);
+	a_store(&t->killlock, 0);
+	return r;
 }