nagios: add meta.{changelog,mainProgram}

pkgs/servers/monitoring/nagios/default.nix

···
   meta = {
     description = "A host, service and network monitoring program";
     homepage = "https://www.nagios.org/";
+    changelog = "https://github.com/NagiosEnterprises/nagioscore/blob/nagios-${version}/Changelog";
     license = lib.licenses.gpl2;
     platforms = lib.platforms.linux;
+    mainProgram = "nagios";
     maintainers = with lib.maintainers; [ immae thoughtpolice relrod ];
   };
 }