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.

modules/language.nix

@@ -0,0 +1,7 @@
+# Language settings
+{ ... }:
+
+{
+  # Select internationalisation properties.
+  i18n.defaultLocale = "en_US.UTF-8";
+}