porthos: split into modules

I have separated the modules into host-specific settings, and generic
settings that ought to be shared by every host.

I only have the 'porthos' host for now, but intend to also add my laptop
'aramis' at some point to this repository.

flake.nix

@@ -19,7 +19,7 @@
               else throw "Refusing to build from a dirty Git tree!";
           })
           { nixpkgs.overlays = [ nur.overlay ]; }
-          ./configuration.nix
+          ./porthos.nix
         ];
     };
   };