Patch #401196: IPv6 extensions to the socket module.

New functions getnameinfo, getaddrinfo. New exceptions socket.gaierror,
socket.herror. Various new constants, in particular AF_INET6 and error
codes and parameters for getaddrinfo.
AF_INET6 support in setipaddr, makesockaddr, getsockaddr, getsockaddrlen,
gethost_common, PySocket_gethostbyaddr.