Add RFC3542-style checksum compuation on raw, IPv6 sockets

This patch adds support for RFC3542-style checksum computation on raw,
IPv6 sockets via the IPV6_CHECKSUM socket option.

This allows the development of application-layer utilities such as
ping6 which are unable to compute the raw packet checksum without a
prior knowledge of the source address selection.

src/include/lwip/raw.h

@@ -97,6 +97,11 @@ struct raw_pcb {
   } recv;
   /* user-supplied argument for the recv callback */
   void *recv_arg;
+#if LWIP_IPV6
+  /* fields for handling checksum computations as per RFC3542. */
+  u8_t  chksum_reqd;
+  u16_t chksum_offset;
+#endif
 };
 
 /* The following functions is the application layer interface to the

src/include/lwip/sockets.h

@@ -230,7 +230,8 @@ struct linger {
 /*
  * Options for level IPPROTO_IPV6
  */
-#define IPV6_V6ONLY 27 /* RFC3493: boolean control to restrict AF_INET6 sockets to IPv6 communications only. */
+#define IPV6_CHECKSUM       7  /* RFC3542: calculate and insert the ICMPv6 checksum for raw sockets. */
+#define IPV6_V6ONLY         27 /* RFC3493: boolean control to restrict AF_INET6 sockets to IPv6 communications only. */
 #endif /* LWIP_IPV6 */
 
 #if LWIP_UDP && LWIP_UDPLITE