nixos systemPackages: rework default outputs

- Now `pkg.outputUnspecified = true` but this attribute is missing in
  every output, so we can recognize whether the user chose or not.
  If (s)he didn't choose, we put `pkg.bin or pkg.out or pkg` into
  `systemPackages`.
- `outputsToLink` is replaced by `extraOutputsToLink`.
  We add extra outputs *regardless* of whether the user chose anything.
  It's mainly meant for outputs with docs and debug symbols.
- Note that as a result, some libraries will disappear from system path.