zabbix-server: Add PID file to systemd config.

This is to avoid (in some cases) constant restarting of the Zabbix server, which
causes odds bugs and crashes in the exit handler (if it's too early during
startup).

Signed-off-by: aszlig <aszlig@redmoonstudios.org>

modules/services/monitoring/zabbix-server.nix

@@ -100,6 +100,7 @@ in
         serviceConfig.Type = "forking";
         serviceConfig.Restart = "always";
         serviceConfig.RestartSec = 2;
+        serviceConfig.PIDFile = pidFile;
       };
 
   };