nixos: introduce system.nixosLabel option and use it where appropriate

Setting nixosVersion to something custom is useful for meaningful GRUB
menus and /nix/store paths, but actuallly changing it rebulids the
whole system path (because of `nixos-version` script and manual
pages). Also, changing it is not a particularly good idea because you
can then be differentitated from other NixOS users by a lot of
programs that read /etc/os-release.

This patch introduces an alternative option that does all you want
from nixosVersion, but rebuilds only the very top system level and
/etc while using your label in the names of system /nix/store paths,
GRUB and other boot loaders' menus, getty greetings and so on.

nixos/modules/system/activation/top-level.nix

@@ -67,7 +67,7 @@ let
 
       echo -n "$configurationName" > $out/configuration-name
       echo -n "systemd ${toString config.systemd.package.interfaceVersion}" > $out/init-interface-version
-      echo -n "$nixosVersion" > $out/nixos-version
+      echo -n "$nixosLabel" > $out/nixos-version
       echo -n "$system" > $out/system
 
       mkdir $out/fine-tune
@@ -101,7 +101,7 @@ let
     if [] == failed then pkgs.stdenv.mkDerivation {
       name = let hn = config.networking.hostName;
                  nn = if (hn != "") then hn else "unnamed";
-          in "nixos-system-${nn}-${config.system.nixosVersion}";
+          in "nixos-system-${nn}-${config.system.nixosLabel}";
       preferLocalBuild = true;
       allowSubstitutes = false;
       buildCommand = systemBuilder;
@@ -115,7 +115,7 @@ let
         config.system.build.installBootLoader
         or "echo 'Warning: do not know how to make this configuration bootable; please enable a boot loader.' 1>&2; true";
       activationScript = config.system.activationScripts.script;
-      nixosVersion = config.system.nixosVersion;
+      nixosLabel = config.system.nixosLabel;
 
       configurationName = config.boot.loader.grub.configurationName;