rework built-in topics again

Move details of built-in topics out of the DDSI core (so the only hooks
remain).  For this, rtps_term had to be split, so now it is "stop"
followed by "fini".

Add a notion of local writers that are not bound to a participant ("local
orphans"), so that the local built-in topic writers can be created during
initialization.  This eliminates the "builtin" participant.  This
uncovered in inconsistency in the unit tests: on the one hand, a newly
created participant is expected to have no child entities; on the other
hand, the built-in topics were expected to be returned by find_topic ...
This inconsistency has been resolved by creating them lazily and
accepting that find_topic can't return them until they have been
created.  Special code was in place in dds_create_reader anyway, so it
is not expected to have any real consequence for applications.

Use a special WHC implementation that regenerates the data on the fly
using the internal discovery tables of DDSI, so that the samples are only
stored by readers.  This eliminates the memory overhead of that existed
previously when the WHC of the writers stored the data.

No longer return topic name and type name in the built-in topics, they
have been extracted already and are not accessible through the normal
interface but do cause problems when comparing QoS.

Signed-off-by: Erik Boasson <eb@ilities.com>

src/core/ddsc/include/ddsc/dds.h

@@ -1777,7 +1777,7 @@ dds_write_flush(
  * @returns A dds_return_t indicating success or failure.
  */
 _Pre_satisfies_((writer & DDS_ENTITY_KIND_MASK) == DDS_KIND_WRITER)
-DDS_EXPORT int
+DDS_EXPORT dds_return_t
 dds_writecdr(
         dds_entity_t writer,
         struct ddsi_serdata *serdata);