Have a go at fixing various outstanding portability issues in code that

Have a go at fixing various outstanding portability issues in code that
was modified for IPv6.  Use a robust definition of struct sockaddr_storage,
do a proper configure test to see if ss_len exists, don't assume that
getnameinfo() will handle AF_UNIX sockets, don't trust getaddrinfo to
return the protocol we ask for, etc.  This incorporates several outstanding
patches from Kurt Roeckx, but I'm to blame for anything that doesn't
work ...