Is sl unfree or too free ? (licensing issue)

Hello everyone,

It's been some time since I discovered NixOS and I like it a lot. Nevertheless, I value software freedom so I tend to avoid the ```allowUnfree``` nuclear switch like the plague. Its existence makes perfect sense though, It's just that when I wanted to install [sl](https://github.com/NixOS/nixpkgs/blob/master/pkgs/tools/misc/sl/default.nix), Nix courageously died, saving me from an unfree software threat!

After investigating, I found that @viric (Thank you for adding the package) had marked it as unfree because it had no standalone license in 26ec1abf41f3873d1e6e292d673df7344b8b432c. With respect, there has to be another solution.

[Toyoda Masashi's short license](https://github.com/mtoyoda/sl/blob/master/LICENSE) probably isn't alone, and we've got no canonical webpage to link to for the text of this kind of license, so I think a generic label which doesn't trigger [stdenv](https://github.com/NixOS/nixpkgs/blob/master/pkgs/stdenv/generic/default.nix#L81)'s wrath is needed.

Louis