Codechange: mark chunks that are not stored as CH_READONLY

This makes it easier to spot chunks that have a save_proc that
is a nullptr, but also prevents confusion, where it looks like
the CH_ type of a chunk has influence on how it is being read.
It is not, it is only used for saving.

src/saveload/engine_sl.cpp

@@ -196,7 +196,7 @@ static void Load_EIDS()
 static const ChunkHandler engine_chunk_handlers[] = {
 	{ 'EIDS', Save_EIDS, Load_EIDS, nullptr, nullptr, CH_ARRAY },
 	{ 'ENGN', Save_ENGN, Load_ENGN, nullptr, nullptr, CH_ARRAY },
-	{ 'ENGS', nullptr,   Load_ENGS, nullptr, nullptr, CH_RIFF  },
+	{ 'ENGS', nullptr,   Load_ENGS, nullptr, nullptr, CH_READONLY  },
 };
 
 extern const ChunkHandlerTable _engine_chunk_handlers(engine_chunk_handlers);