Make <nix/fetchurl.nix> a builtin builder

This ensures that 1) the derivation doesn't change when Nix changes;
2) the derivation closure doesn't contain Nix and its dependencies; 3)
we don't have to rely on ugly chroot hacks.

src/libstore/builtins.cc

@@ -0,0 +1,24 @@
+#include "builtins.hh"
+#include "download.hh"
+
+namespace nix {
+
+void builtinFetchurl(const BasicDerivation & drv)
+{
+    auto url = drv.env.find("url");
+    if (url == drv.env.end()) throw Error("attribute ‘url’ missing");
+    printMsg(lvlInfo, format("downloading ‘%1%’...") % url->second);
+    auto data = downloadFile(url->second); // FIXME: show progress
+
+    auto out = drv.env.find("out");
+    if (out == drv.env.end()) throw Error("attribute ‘url’ missing");
+    writeFile(out->second, data.data);
+
+    auto executable = drv.env.find("out");
+    if (executable != drv.env.end() && executable->second == "1") {
+        if (chmod(out->second.c_str(), 0755) == -1)
+            throw SysError(format("making ‘%1%’ executable") % out->second);
+    }
+}
+
+}