nixos: create 'modules/nixos' folder

Let's consolidate all modules under one path, so that NixOS,
home-manager, and nix-darwin (if I ever end up using it down the line)
would go under the same folder.

modules/nixos/system/users/default.nix

@@ -0,0 +1,51 @@
+# User setup
+{ config, lib, pkgs, ... }:
+let
+  secrets = config.age.secrets;
+  cfg = config.my.system.users;
+  groupExists = grp: builtins.hasAttr grp config.users.groups;
+  groupsIfExist = builtins.filter groupExists;
+in
+{
+  options.my.system.users = with lib; {
+    enable = my.mkDisableOption "user configuration";
+  };
+
+  config = lib.mkIf cfg.enable {
+    users = {
+      mutableUsers = false; # I want it to be declarative.
+
+      users = {
+        root = {
+          hashedPasswordFile = secrets."users/root/hashed-password".path;
+        };
+
+        ${config.my.user.name} = {
+          hashedPasswordFile = secrets."users/ambroisie/hashed-password".path;
+          description = "Bruno BELANYI";
+          isNormalUser = true;
+          shell = pkgs.zsh;
+          extraGroups = groupsIfExist [
+            "audio" # sound control
+            "docker" # usage of `docker` socket
+            "media" # access to media files
+            "networkmanager" # wireless configuration
+            "plugdev" # usage of ZSA keyboard tools
+            "podman" # usage of `podman` socket
+            "video" # screen control
+            "wheel" # `sudo` for the user.
+          ];
+          openssh.authorizedKeys.keys = with builtins;
+            let
+              keyDir = ./ssh;
+              contents = readDir keyDir;
+              names = attrNames contents;
+              files = filter (name: contents.${name} == "regular") names;
+              keys = map (basename: readFile (keyDir + "/${basename}")) files;
+            in
+            keys;
+        };
+      };
+    };
+  };
+}