Refactor gnupg+pinentry installation

Ensure pinentry-gtk-2 is used so we can theme its prompt, and ensure
that gpg respects XDG conventions.

modules/shell/gnupg.nix

@@ -1,13 +1,32 @@
 { config, lib, pkgs, ... }:
 
 {
-  environment.systemPackages = with pkgs; [
+  environment = {
+    extraInit = ''
+      export GNUPGHOME="$XDG_CONFIG_HOME/gnupg"
+      if [ ! -d "$GNUPGHOME" ]; then
+         mkdir -p "$GNUPGHOME" -m 700
+      fi
+    '';
+
+    systemPackages = with pkgs; [
       gnupg
       pinentry
     ];
+  };
 
   programs.gnupg.agent = {
     enable = true;
     enableSSHSupport = true;
   };
+
+  systemd.user.services.gpg-agent.serviceConfig.ExecStart = [
+    "" ''
+       ${pkgs.gnupg}/bin/gpg-agent \
+            --supervised \
+            --allow-emacs-pinentry \
+            --default-cache-ttl 1200 \
+            --pinentry-program ${pkgs.pinentry}/bin/pinentry-gtk-2
+       ''
+  ];
 }