simplify cancellation point handling

we take advantage of the fact that unless self->cancelpt is 1,
cancellation cannot happen. so just increment it by 2 to temporarily
block cancellation. this drops pthread_create.o well under 1k.

src/thread/pthread_create.c

@@ -63,19 +63,8 @@ static void cancel_handler(int sig, siginfo_t *si, void *ctx)
 static void cancelpt(int x)
 {
 	struct pthread *self = __pthread_self();
-	switch (x) {
-	case 1:
-		self->cancelpoint++;
-	case 0:
-		if (self->cancel && self->cancelpoint==1 && !self->canceldisable)
-			docancel(self);
-		break;
-	case -1:
-		self->cancelpoint--;
-		break;
-	default:
-		self->canceldisable += x;
-	}
+	if ((self->cancelpoint+=x)==1 && self->cancel
+		&& x<2U && !self->canceldisable) docancel(self);
 }
 
 static void init_threads()

src/thread/pthread_setcancelstate.c

@@ -3,8 +3,8 @@
 int pthread_setcancelstate(int new, int *old)
 {
 	struct pthread *self = pthread_self();
-	if (old) *old = self->canceldisable & 1;
-	if ((unsigned)new > 1) return EINVAL;
-	self->canceldisable = (self->canceldisable & ~1) | new;
+	if (old) *old = self->canceldisable;
+	if (new > 1U) return EINVAL;
+	self->canceldisable = new;
 	return 0;
 }