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/drivers/drv_orb_dev.h

@@ -93,4 +93,7 @@
 /** Get the minimum interval at which the topic can be seen to be updated for this subscription */
 #define ORBIOCGETINTERVAL	_ORBIOC(16)
 
+/** Check whether the topic is published, sets *(unsigned long *)arg to 1 if published, 0 otherwise */
+#define ORBIOCISPUBLISHED	_ORBIOC(17)
+
 #endif /* _DRV_UORB_H */