more rdataslab refactoring

- there are now two functions for getting rdataslab size:
  dns_rdataslab_size() is for full slabs and dns_rdataslab_sizeraw()
  for raw slabs. there is no longer a need for a reservelen parameter.
- dns_rdataslab_count() also no longer takes a reservelen parameter.
  (currently it's never used for raw slabs, so there is no _countraw()
  function.)
- dns_rdataslab_rdatasize() has been removed, because
  dns_rdataslab_sizeraw() can do the same thing.
- dns_rdataslab_merge() and dns_rdataslab_subtract() both take
  slabheader parameters instead of character buffers, and the
  reservelen parameter has been removed.

lib/dns/qpcache.c

@@ -2286,8 +2286,7 @@ expiredata(dns_db_t *db, dns_dbnode_t *node, void *data) {
 static size_t
 rdataset_size(dns_slabheader_t *header) {
 	if (EXISTS(header)) {
-		return dns_rdataslab_size((unsigned char *)header,
-					  sizeof(*header));
+		return dns_rdataslab_size(header);
 	}
 
 	return sizeof(*header);