Default-generated config is a bit of a mess

User config file generated by default is a bit of a mess rn:

```
$ cabal --config=cabal-config user-config init
$ cat cabal-config
```

E.g. there's both a `compiler` and a `with-compiler` field, `write-ghc-environment-files` lacks a default, there are things like `exact-configuration` that should be hidden, etc. Looks like this started when `new-install` landed.

We should also think about putting more stuff into separate sections.

Start by running `cabal --config=cabal-config user-config init` and inspecting the generated `cabal-config`. Compare the `compiler`, `with-compiler`, `write-ghc-environment-files`, and `exact-configuration` fields to determine which defaults and visibility are appropriate. Done means the generated configuration is organized into sensible sections without redundant or inappropriate fields.

Written by the indexing model from the issue text.