Fix debug logging

Resolves issue #6946

Problem was in:
```
static DebugFile debugFile;
```
It's impossible to use dynamic filename(FILENAME["debug"] in that object because it's statically allocated.
So, i add new DebugLog method ``` init ```. It set DebugLog filename after static allocation.

I am not familiar with C++ syntax. Please check my systax:
```
{
  DebugFile();
  ~DebugFile();
  void init(std::string filename);
  ...
}
void DebugLog::init(std::string filename)
{
  ...
}
```
Tested. Work on Linux.