Implement opt-in for anonymous build reporting

(Imported from [Trac #732](http://hackage.haskell.org/trac/hackage/ticket/732), reported by @dcoutts on 2010-09-01)

We want people to send in anonymous build reports. We also want to respect people's privacy. The suggestion is that we should use an opt-in system but make it very easy to opt in.

Currently most users have a config file that contains no value set for the config file fields that control the build reporting feature. The suggestion is that when the build reporting config items are not set then we should interactively prompt users like so:

$ cabal update

Cabal can send build reports back to to the package server.

Build reporting helps to improve package quality. The information

sent back is minimal and it does not identify your computer.

For more details see: http://haskell.org/cabal/reporting

Enable automatic anonymous reporting of package builds? [y/n/?]

Downloading the latest package list from hackage.haskell.org

We would then rewrite the config file with the updated setting, either on or off. That will require that we correctly preserve settings and comments in the config file.