Revert "nixos: rename system.{stateVersion,defaultChannel} -> system.nixos.\1"

This reverts commit 095fe5b43d.

Pointless renames considered harmful. All they do is force people to
spend extra work updating their configs for no benefit, and hindering
the ability to switch between unstable and stable versions of NixOS.

Like, what was the value of having the "nixos." there? I mean, by
definition anything in a NixOS module has something to do with NixOS...

nixos/modules/misc/version.nix

@@ -12,29 +12,29 @@ in
 
 {
 
-  options.system.nixos = {
+  options.system = {
 
-    version = mkOption {
+    nixos.version = mkOption {
       internal = true;
       type = types.str;
       description = "The full NixOS version (e.g. <literal>16.03.1160.f2d4ee1</literal>).";
     };
 
-    release = mkOption {
+    nixos.release = mkOption {
       readOnly = true;
       type = types.str;
       default = trivial.release;
       description = "The NixOS release (e.g. <literal>16.03</literal>).";
     };
 
-    versionSuffix = mkOption {
+    nixos.versionSuffix = mkOption {
       internal = true;
       type = types.str;
       default = trivial.versionSuffix;
       description = "The NixOS version suffix (e.g. <literal>1160.f2d4ee1</literal>).";
     };
 
-    revision = mkOption {
+    nixos.revision = mkOption {
       internal = true;
       type = types.str;
       default = if pathIsDirectory gitRepo then commitIdFromGitRepo gitRepo
@@ -43,7 +43,7 @@ in
       description = "The Git revision from which this NixOS configuration was built.";
     };
 
-    codeName = mkOption {
+    nixos.codeName = mkOption {
       readOnly = true;
       type = types.str;
       description = "The NixOS release code name (e.g. <literal>Emu</literal>).";
@@ -76,8 +76,8 @@ in
 
   config = {
 
-    warnings = lib.optional (options.system.nixos.stateVersion.highestPrio > 1000)
-      "You don't have `system.nixos.stateVersion` explicitly set. Expect things to break.";
+    warnings = lib.optional (options.system.stateVersion.highestPrio > 1000)
+      "You don't have `system.stateVersion` explicitly set. Expect things to break.";
 
     system.nixos = {
       # These defaults are set here rather than up there so that