systemd: Support reload scripts

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

@@ -230,6 +230,15 @@ in rec {
       '';
     };
 
+    reload = mkOption {
+      type = types.lines;
+      default = "";
+      description = ''
+        Shell commands executed when the service's main process
+        is reloaded.
+      '';
+    };
+
     preStop = mkOption {
       type = types.lines;
       default = "";

nixos/modules/system/boot/systemd.nix

@@ -245,6 +245,12 @@ let
               ${config.postStart}
             '';
           })
+        (mkIf (config.reload != "")
+          { serviceConfig.ExecReload = makeJobScript "${name}-reload" ''
+              #! ${pkgs.stdenv.shell} -e
+              ${config.reload}
+            '';
+          })
         (mkIf (config.preStop != "")
           { serviceConfig.ExecStop = makeJobScript "${name}-pre-stop" ''
               #! ${pkgs.stdenv.shell} -e