Allow a unit to declare "triggers" that force a restart

The triggers are just arbitrary strings that are included in the unit
under X-Restart-Triggers.  The idea is that if they change between
reconfigurations, switch-to-configuration will restart the unit
because its store path changed.  This is mostly useful for services
that implicitly depend on generated files in /etc.  Thus you can say

  restartTriggers = [ confFile ];

where ‘confFile’ is the derivation that generated the /etc file in
question.

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

@@ -83,6 +83,15 @@ rec {
       '';
     };
 
+    restartTriggers = mkOption {
+      default = [];
+      description = ''
+        An arbitrary list of items such as derivations.  If any item
+        in the list changes between reconfigurations, the service will
+        be restarted.
+      '';
+    };
+
   };