Update spec library/socket/socket/gethostbyname.

Add two new specs to verify that addresses in address list returned
by Socket.gethosbyname are in 'pack' format (one spec for IPv4
addresses an the other for IPv6 ones).
Update Socket.gethostbyname to respect two new specs.

Socket.gethostbyname is updated for all ruby versions.

Socket.sockaddr_in is used to transform addresses got from
Socket.getaddrinfo to packed strings. A new structure is added for IPv6
addresses so they can be transformed properly.