Move debug.txt after it grows too big (#8904)

Before opening the file for writing, its file size is tested. If it exceeds 50 MB, it is moved to debut.txt.1, otherwise the log is appended to the old messages. An old debut.txt.1 is removed if it already exists.

src/log.h

@@ -165,7 +165,7 @@ private:
 
 class FileLogOutput : public ICombinedLogOutput {
 public:
-	void open(const std::string &filename);
+	void setFile(const std::string &filename, s64 file_size_max);
 
 	void logRaw(LogLevel lev, const std::string &line)
 	{