introduce site overlays

site overlays are place in sysconfdir/overlays and take
precedence over distribution overlays with the same name.

Every `wwctl overlay` command changing overlays will create
an site overlay. distribution overlays can't be deleted or
modified with wwctl.

Signed-off-by: Christian Goll <cgoll@suse.com>

internal/app/wwctl/overlay/chown/main.go

@@ -34,8 +34,11 @@ func CobraRunE(cmd *cobra.Command, args []string) error {
 	} else {
 		gid = -1
 	}
-
-	overlaySourceDir = overlay.OverlaySourceDir(overlayName)
+	err = overlay.CreateSiteOverlay(overlayName)
+	if err != nil {
+		return err
+	}
+	overlaySourceDir, _ = overlay.OverlaySourceDir(overlayName)
 
 	if !util.IsDir(overlaySourceDir) {
 		return fmt.Errorf("overlay does not exist: %s", overlayName)