`tool --version` now gets additional line:
Packaged by https://NixOS.org

- default coreutils is stripped of /share/ (11 -> 2 MiB)
- coreutils-full retains /share/ and adds openssl for faster *sum tools
- NixOS systemPackages contains coreutils-full
- *Support parameter defaults are moved inside
  (it seemed confusing to have `? false` and "at once" with `? isLinux`)

Closure considerations:
+ typical build-time closure will get lighter by ~9 MiB
- typical closure of NixOS installation will grow by ~2 MiB,
  due to referring to both versions.  I think it would be possible to
  re-use most of the utils between the two versions, but the expression
  would get much more complex.

I considered having stdenv with minimal coreutils and the default
`coreutils` attribute being full, but it turned out there were too many
trivial references in nixpkgs, so it didn't seem easy to keep rebuild
impact of openssl from growing significantly.