orb_advert_t changed to void * and checks changed to nullptr

The existing orb_advert_t use thoughout the code sometimes tries
to treat it as a file descriptor and there are checks for < 0
and ::close calls on orb_advert_t types which is an invalid use
of an object pointer, which is what orb_advert_t really is.

Initially I had changed the -1 initializations to 0 but it was
suggested that this should be nullptr. That was a good recommendation
but the definition of orb_advert_t had to change to void * because
you cannot initialize a uintptr_t as nullptr.

Signed-off-by: Mark Charlebois <charlebm@gmail.com>

src/modules/uORB/uORBManager_posix.cpp

@@ -94,7 +94,7 @@ orb_advert_t uORB::Manager::orb_advertise_multi(const struct orb_metadata *meta,
   fd = node_open(PUBSUB, meta, data, true, instance, priority);
   if (fd == ERROR) {
     warnx("node_open as advertiser failed.");
-    return 0;
+    return nullptr;
   }
 
   /* get the advertiser handle and close the node */
@@ -102,14 +102,14 @@ orb_advert_t uORB::Manager::orb_advertise_multi(const struct orb_metadata *meta,
   px4_close(fd);
   if (result == ERROR) {
     warnx("px4_ioctl ORBIOCGADVERTISER  failed. fd = %d", fd);
-    return 0;
+    return nullptr;
   }
 
   /* the advertiser must perform an initial publish to initialise the object */
   result = orb_publish(meta, advertiser, data);
   if (result == ERROR) {
     warnx("orb_publish failed");
-    return 0;
+    return nullptr;
   }
 
   return advertiser;