This commit address two issues with sockaddr struct implementations for
IPv6:
1) struct sockaddr_in6 should have 32-bit unsigned field sin6_scope_id
as specified in Section 3.4 of RFC 3493 (Basic Socket Interface
Extensions for IPv6)
2) struct sockaddr is not extended in IPv6 to contain space for
struct sockaddr_in6. Applications should be using struct
sockaddr_storage when needing generic storage. This removes the
extra bytes added when LWIP_IPV6 is defined
d850efdd08