[Nix-dev] restricted downloads

[Yury G. Kudryashov]

Hi!

I've just commited support for "restricted downloads" in fetchurl. The idea 
is that you can write
src = fetchurl {
  url = "http://restricted.noltd/download.html";
  name = "my_restricted_src.zip";
  restricted = true;
  sha256 = "myhash";
};

When a user will try to build this package, fetchurl will show message like 
"go to ${url}, download ${name} and add it using nix-store --add-fixed 
sha256".

I hope that this commit breaks nothing. If not, please ping me before 
reverting. I will be on-line most time tomorrow.

P.S.: What do you think about printing this message in bold or in red?