HNA set

A HNA parser function is registered with the packet parser at startup. This function maintains the HNA set. All received HNA information that is valid is kept in this data-set. This means that multiple HNA entries can exists for the same network if multiple nodes announce themselves as gateways to the same network. If this happens, only the gateway closest, in terms of hop-count, is to be added to the routing table. All HNA entries are however, to be registered and processed. Most of this code is kept in the src/hna_set.c file.