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/settings.cpp

@@ -2105,7 +2105,7 @@ static void Save_PATS()
 }
 
 static const ChunkHandler setting_chunk_handlers[] = {
-	{ 'OPTS', nullptr,   Load_OPTS, nullptr, nullptr,    CH_RIFF  },
+	{ 'OPTS', nullptr,   Load_OPTS, nullptr, nullptr,    CH_READONLY },
 	{ 'PATS', Save_PATS, Load_PATS, nullptr, Check_PATS, CH_ARRAY },
 };