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/services/ssh-server/default.nix

@@ -0,0 +1,26 @@
+# An SSH server, using 'mosh'
+{ config, lib, ... }:
+let
+  cfg = config.my.services.ssh-server;
+in
+{
+  options.my.services.ssh-server = {
+    enable = lib.mkEnableOption "SSH Server using 'mosh'";
+  };
+
+  config = lib.mkIf cfg.enable {
+    services.openssh = {
+      # Enable the OpenSSH daemon.
+      enable = true;
+
+      settings = {
+        # Be more secure
+        PermitRootLogin = "no";
+        PasswordAuthentication = false;
+      };
+    };
+
+    # Opens the relevant UDP ports.
+    programs.mosh.enable = true;
+  };
+}