Update Socket.gethostbyname to respect two new specs.

Socket.gethostbyname is updated for all ruby versions.

* rakelib/platform.rake:
  Add new structure `sockaddr_in6`

* lib/**/socket.rb :
  Socket.sockaddr_in is used to transform addresses got
  from Socket.getaddrinfo to packed strings. Platform
  configuration is then used to determine offset and size
  of IP address in sockaddr string. `sockaddr_in` and
  `sockaddr_in6` platform structures are used for this
  purpose.
Merge branch 'socket.gethostbyname' of github.com:sdaubert/rubinius into socket.gethostbyname