Feature: Persistant rotation of numbered auto/netsave after restart (#9397)

It was always starting from 0 on openttd restart.
Now the most recent auto/netsave number will be used as a base to generate the next filename.

src/fios.h

@@ -125,4 +125,16 @@ std::string FiosMakeSavegameName(const char *name);
 
 FiosType FiosGetSavegameListCallback(SaveLoadOperation fop, const std::string &file, const char *ext, char *title, const char *last);
 
+/**
+ * A savegame name automatically numbered.
+ */
+struct FiosNumberedSaveName {
+	FiosNumberedSaveName(const std::string &prefix);
+	std::string Filename();
+	std::string Extension();
+private:
+	std::string prefix;
+	int number;
+};
+
 #endif /* FIOS_H */