nd6: some work on basic RFC 4861 compliance

The current ND implementation does not yet implement the most basic
required ('MUST') checks for message validation and generation.

- implement some of the required checks for message validation;
- document the remaining missing message validation checks;
- hardcode the hop limit of Neighbor Discovery messages rather than
  having it depend on lwIP configuration which, if changed, would
  cause all of ND to cease working.

src/include/lwip/priv/nd6_priv.h

@@ -118,6 +118,8 @@ enum nd6_neighbor_cache_entry_state {
   ND6_PROBE
 };
 
+#define ND6_HOPLIM 255 /* maximum hop limit, required in all ND packets */
+
 #define ND6_2HRS 7200 /* two hours, expressed in number of seconds */
 
 /* Router tables. */