w3m: Add a variant without graphics support and use for NixOS manual

This gets rid of various graphics libraries from the minimal installer.

nixos/modules/services/misc/nixos-manual.nix

···
 
     services.nixosManual.browser = mkOption {
       type = types.path;
-      default = "${pkgs.w3m-nox}/bin/w3m";
+      default = "${pkgs.w3m-nographics}/bin/w3m";
       description = ''
         Browser used to show the manual.
       '';

pkgs/top-level/all-packages.nix

···
     imlib2 = imlib2-nox;
   };
 
+  # Version without X11 or graphics
+  w3m-nographics = w3m.override {
+    x11Support = false;
+    graphicsSupport = false;
+  };
+
   # Version for batch text processing, not a good browser
   w3m-batch = w3m.override {
     graphicsSupport = false;