orb_exists: change semantics from (is published or subscribed) to (is published)

Existing users of orb_exists:
- logger (dynamic subscribe to multi-instances)
- mavlink (orb subscription)
- sdlog2
- preflightcheck (check for home_position)
- wait_for_topic shell command (it's not used)
- orb_group_count() (sensors: dynamic sensor addition)

All use-cases benefit from the changed semantics: they are really only
interested if there is a publisher, not another subscriber.

src/modules/uORB/uORBDevices.cpp

@@ -170,6 +170,10 @@ uORB::DeviceNode::open(device::file_t *filp)
 		return ret;
 	}
 
+	if (FILE_FLAGS(filp) == 0) {
+		return CDev::open(filp);
+	}
+
 	/* can only be pub or sub, not both */
 	return -EINVAL;
 }
@@ -405,6 +409,11 @@ uORB::DeviceNode::ioctl(device::file_t *filp, int cmd, unsigned long arg)
 
 		return OK;
 
+	case ORBIOCISPUBLISHED:
+		*(unsigned long *)arg = _published;
+
+		return OK;
+
 	default:
 		/* give it to the superclass */
 		return CDev::ioctl(filp, cmd, arg);