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_create.c

@@ -27,7 +27,9 @@ void __pthread_unwind_next(struct __ptcb *cb)
 	__lock(&self->exitlock);
 
 	/* Mark this thread dead before decrementing count */
+	__lock(&self->killlock);
 	self->dead = 1;
+	a_store(&self->killlock, 0);
 
 	do n = libc.threads_minus_1;
 	while (n && a_cas(&libc.threads_minus_1, n, n-1)!=n);