libthread: fix nbrecvul, recvul to match man page, Plan 9

They return 0 on failure, not -1.
Bug introduced in my original libthread-for-Unix code.

Fixes #230.

src/libthread/channel.c

@@ -397,7 +397,7 @@ chanrecvul(Channel *c)
 
 	if(_chanop(c, CHANRCV, &val, 1) > 0)
 		return val;
-	return -1;
+	return 0;
 }
 
 int
@@ -413,5 +413,5 @@ channbrecvul(Channel *c)
 
 	if(_chanop(c, CHANRCV, &val, 0) > 0)
 		return val;
-	return -1;
+	return 0;
 }