fix signal masking race in pthread_create with priority attributes

if the parent thread was able to set the new thread's priority before
it reached the check for 'startlock', the new thread failed to restore
its signal mask and thus ran with all signals blocked.

concept for patch by Sergei, who reported the issue; unnecessary
changes were removed and comments added since the whole 'startlock'
thing is non-idiomatic and confusing. eventually it should be replaced
with use of idiomatic synchronization primitives.

src/thread/pthread_create.c

@@ -131,9 +131,14 @@ void __do_cleanup_pop(struct __ptcb *cb)
 static int start(void *p)
 {
 	pthread_t self = p;
+	/* States for startlock:
+	 * 0 = no need for start sync
+	 * 1 = waiting for parent to do work
+	 * 2 = failure in parent, child must abort
+	 * 3 = success in parent, child must restore sigmask */
 	if (self->startlock[0]) {
 		__wait(self->startlock, 0, 1, 1);
-		if (self->startlock[0]) {
+		if (self->startlock[0] == 2) {
 			self->detached = 2;
 			pthread_exit(0);
 		}
@@ -295,7 +300,7 @@ int __pthread_create(pthread_t *restrict res, const pthread_attr_t *restrict att
 	if (do_sched) {
 		ret = __syscall(SYS_sched_setscheduler, new->tid,
 			attr._a_policy, &attr._a_prio);
-		a_store(new->startlock, ret<0 ? 2 : 0);
+		a_store(new->startlock, ret<0 ? 2 : 3);
 		__wake(new->startlock, 1, 1);
 		if (ret < 0) return -ret;
 	}