The thing is that on some platforms (e.g. Mac OS X) sockaddr_in / sockaddr_in6
contain a non-standard field (sin_len / sin6_len), that must be set.

The problem was that only standard fields were set, thus getnameinfo() returned
EAI_SYSTEM instead of EAI_NONAME.

The fix is to introduce configure-time checks (for GNU auto-tools and CMake) for
those additional fields and to set them if they are available.