Warn if -v7.3 format is used for load/save (bug #45706). * load-save.cc (Fload, Fsave): Document that -v7.3 format is not implemented. * load-save.cc (load_save_system::parse_save_options): Detect '-v7.3' option and issue an error() that format is not implemented. * load-save.cc (load_save_system::load): Detect '-v7.3' option and issue an error() that format is not implemented. Re-order if/elseif tree for clarity.