Add option environment.enableDebugInfo

This makes the debug outputs of packages that have them available to
programs like gdb.

nixos/modules/config/debug-info.nix

···
+{ config, lib, ... }:
+
+with lib;
+
+{
+
+  options = {
+
+    environment.enableDebugInfo = mkOption {
+      type = types.bool;
+      default = false;
+      description = ''
+        Some NixOS packages provide debug symbols. However, these are
+        not included in the system closure by default to save disk
+        space. Enabling this option causes the debug symbols to appear
+        in <filename>/run/current-system/sw/lib/debug/.build-id</filename>,
+        where tools such as <command>gdb</command> can find them.
+        If you need debug symbols for a package that doesn't
+        provide them by default, you can enable them as follows:
+        <!-- FIXME: ugly, see #10721 -->
+        <programlisting>
+        nixpkgs.config.packageOverrides = pkgs: {
+          hello = overrideDerivation pkgs.hello (attrs: {
+            outputs = attrs.outputs or ["out"] ++ ["debug"];
+            buildInputs = attrs.buildInputs ++ [&lt;nixpkgs/pkgs/build-support/setup-hooks/separate-debug-info.sh>];
+          });
+        };
+        </programlisting>
+      '';
+    };
+
+  };
+
+
+  config = {
+
+    # FIXME: currently disabled because /lib is already in
+    # environment.pathsToLink, and we can't have both.
+    #environment.pathsToLink = [ "/lib/debug/.build-id" ];
+
+    environment.outputsToLink =
+      optional config.environment.enableDebugInfo "debug";
+
+  };
+
+}

nixos/modules/config/system-path.nix

···
       [ "/bin"
         "/etc/xdg"
         "/info"
-        "/lib" # FIXME: remove
-        #"/lib/debug/.build-id" # enables GDB to find separated debug info
+        "/lib" # FIXME: remove and update debug-info.nix
         "/sbin"
         "/share/applications"
         "/share/desktop-directories"

nixos/modules/module-list.nix

···
 [
+  ./config/debug-info.nix
   ./config/fonts/corefonts.nix
+  ./config/fonts/fontconfig-ultimate.nix
   ./config/fonts/fontconfig.nix
-  ./config/fonts/fontconfig-ultimate.nix
   ./config/fonts/fontdir.nix
   ./config/fonts/fonts.nix
   ./config/fonts/ghostscript.nix
···
   ./config/system-environment.nix
   ./config/system-path.nix
   ./config/timezone.nix
-  ./config/vpnc.nix
   ./config/unix-odbc-drivers.nix
   ./config/users-groups.nix
+  ./config/vpnc.nix
   ./config/zram.nix
   ./hardware/all-firmware.nix
   ./hardware/cpu/amd-microcode.nix

pkgs/tools/package-management/nix/default.nix

···
 
     doInstallCheck = false;
 
+    separateDebugInfo = stdenv.isLinux;
+
     crossAttrs = {
       postUnpack =
         '' export CPATH="${bzip2.crossDrv}/include"