A simple ADC downloader for upcoming darwin awesomeness

Closes #4327

pkgs/build-support/fetchadc/builder.sh

···
+source $stdenv/setup
+
+loginpage=`curl --insecure -s -L -b cookies.txt "$url"`
+
+[[ $loginpage =~ form[^\>]+action=\"([^\"]+)\" ]] && loginurl=${BASH_REMATCH[1]}
+
+curl  --insecure -s --output "$out" -L -b cookies.txt --data "appleId=${adc_user}&accountPassword=${adc_pass}" "https://idmsa.apple.com/IDMSWebAuth/${loginurl}"

pkgs/build-support/fetchadc/default.nix

···
+{ stdenv, curl, adc_user, adc_pass }:
+
+let
+  impureEnvVars = [
+    # We borrow these environment variables from the caller to allow
+    # easy proxy configuration.  This is impure, but a fixed-output
+    # derivation like fetchurl is allowed to do so since its result is
+    # by definition pure.
+    "http_proxy" "https_proxy" "ftp_proxy" "all_proxy" "no_proxy"
+  ];
+in
+
+{ # URL to fetch.
+  url
+
+  # Hash of the downloaded file
+, sha256
+
+, # Additional curl options needed for the download to succeed.
+  curlOpts ? ""
+
+, # Name of the file.  If empty, use the basename of `url' (or of the
+  # first element of `urls').
+  name ? ""
+}:
+
+stdenv.mkDerivation {
+  name    = if name != "" then name else baseNameOf (toString url);
+  builder = ./builder.sh;
+
+  buildInputs = [ curl ];
+
+  meta = {
+    # Password-guarded files from ADC are certainly unfree, as far as we're concerned!
+    license = stdenv.lib.licenses.unfree;
+  };
+
+  outputHashAlgo = "sha256";
+  outputHash     =  sha256;
+  outputHashMode = "flat";
+
+  inherit curlOpts url adc_user adc_pass;
+
+  preferLocalBuild = true;
+}

pkgs/top-level/all-packages.nix

···
     vs = vs90wrapper;
   };
 
+  fetchadc = import ../build-support/fetchadc {
+    inherit curl stdenv;
+    inherit (config) adc_user adc_pass;
+  };
+
   fetchbower = import ../build-support/fetchbower {
     inherit stdenv git;
     inherit (nodePackages) fetch-bower;