Add support for postStop scripts

modules/system/boot/systemd-unit-options.nix

@@ -145,6 +145,15 @@ rec {
       '';
     };
 
+    postStop = mkOption {
+      type = types.string;
+      default = "";
+      description = ''
+        Shell commands executed after the service's main process
+        has exited.
+      '';
+    };
+
     restartIfChanged = mkOption {
       type = types.bool;
       default = true;

modules/system/boot/systemd.nix

@@ -250,6 +250,13 @@ let
             ''}
           ''}
 
+          ${optionalString (def.postStop != "") ''
+            ExecStopPost=${makeJobScript "${name}-poststop.sh" ''
+              #! ${pkgs.stdenv.shell} -e
+              ${def.postStop}
+            ''}
+          ''}
+
           ${attrsToSection def.serviceConfig}
         '';
     };