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/uORBTest_UnitTest.cpp

@@ -192,7 +192,7 @@ int uORBTest::UnitTest::test()
   test_note("try multi-topic support");
 
   int instance0;
-  int pfd0 = orb_advertise_multi(ORB_ID(orb_multitest), &t, &instance0, ORB_PRIO_MAX);
+  orb_advert_t pfd0 = orb_advertise_multi(ORB_ID(orb_multitest), &t, &instance0, ORB_PRIO_MAX);
 
   test_note("advertised");