For now, leave the old implementation under `man-old` attribute.
Small warning: I had a leftover ~/.nix-profile/man from an old package,
which caused man-db's man prefer it and ignore ~/.nix-profile/share/man.
The PATH->MANPATH code just selects the first match for each PATH item.
- 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.
