httpd.service: Support reload

This is useful when ACME has generated a new TLS certificate.

nixos/modules/services/web-servers/apache-httpd/default.nix

···
 
         serviceConfig.ExecStart = "@${httpd}/bin/httpd httpd -f ${httpdConf}";
         serviceConfig.ExecStop = "${httpd}/bin/httpd -f ${httpdConf} -k graceful-stop";
+        serviceConfig.ExecReload = "${httpd}/bin/httpd -f ${httpdConf} -k graceful";
         serviceConfig.Type = "forking";
         serviceConfig.PIDFile = "${mainCfg.stateDir}/httpd.pid";
         serviceConfig.Restart = "always";