localhost redux

I've been having a problem with the pgstat.c localhost change since 7.4,
in that statistics gathering stopped working on my gentoo boxes. I've
not traced it back beyond getaddrinfo called in getaddrinfo_all, and
decided the quick fix was to go back and change pgstat to use 127.0.0.1
instead of localhost.

I don't know if the problem is isolated to just me, but I wanted to
suggest that we use a parameter for that, which can be configured in the
postgresql.conf, with a default value if it's not set, set to
localhost. I'd be happy to write a patch that follows this behavior but
wanted to see if everyone agrees with that line of thinking before doing so.

Gavin