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/printing/default.nix

@@ -0,0 +1,69 @@
+{ config, lib, pkgs, ... }:
+let
+  cfg = config.my.system.printing;
+in
+{
+  options.my.system.printing = with lib; {
+    enable = mkEnableOption "printing configuration";
+
+    papersize = mkOption {
+      type = with types; either str (enum [
+        "a3"
+        "a4"
+        "a5"
+        "b5"
+        "letter"
+        "legal"
+        "executive"
+        "note"
+        "11x17"
+      ]);
+      default = "a4";
+      example = "paper";
+      description = "preferred paper size";
+    };
+
+    usb = {
+      enable = my.mkDisableOption "USB printers";
+    };
+
+    network = {
+      enable = my.mkDisableOption "network printers";
+    };
+  };
+
+  config = lib.mkIf cfg.enable {
+    # Setup CUPS
+    services.printing = {
+      enable = true;
+
+      # Drivers are deprecated, but just in case
+      drivers = with pkgs; [
+        gutenprint # Base set of drivers
+        brlaser # Brother drivers
+
+        # Brother MFC-L3770CDW
+        mfcl3770cdwlpr
+        mfcl3770cdwcupswrapper
+      ];
+    };
+
+    # Setup paper size
+    systemd.services.cups.serviceConfig.Environment = [
+      "PAPERSIZE=${cfg.papersize}"
+    ];
+
+    # Allow using USB printers
+    services.ipp-usb = lib.mkIf cfg.usb.enable {
+      enable = true;
+    };
+
+    # Allow using WiFi printers
+    services.avahi = lib.mkIf cfg.network.enable {
+      enable = true;
+      openFirewall = true;
+      # Allow resolution of '.local' addresses
+      nssmdns = true;
+    };
+  };
+}