Ipv6 network cleanup patch #2.

Here is an updated patch for my ipv6 related changes.

Most important change is to get my cidr support in line with the
patch Andrew Dunstan made.

I have 3 things that might need changing:
- configure now checks inet_ntop() to check if there is ipv6
support, where we don't even use that function anymore now.
Maybe just checking for sockaddr_in6 will be enough?

- pg_hba.conf.sample has ipv6 addresses in them. If you don't
have support for ipv6 it will give an error on that. Not sure
what to do with that.

- Maybe have a getnameinfo_unix(), like getaddrinfo_unix()?

Note that I only tested this on Linux so far.

I also attached a file with the changes in it.

Kurt