New Drive type: overlay.

When active, this drive redirects new and changed files to a different location.
The files in the overlay and normal drive are merged on startup and kept up to date when the game changes something. 
Files in the overlay are priotizedi, if they exists, above the normal files. 
The drive will switch to an overlayed version of the file on the first write! (Not when opening the file in write-mode).
The overlay is capable of creating missing directories.

With the help of DBOVERLAY files, the drive keeps track of files that are present in the normal directory but deleted by the game. 
All changes are preserved between sessions. 

Current design principles/limitations/requirements:
1) All directories that can be used for saving, must exist already in the base before mounting. (they will be created by DOSBox if missing in the overlay)
2) All filenames inside the overlay directories are UPPERCASE and conform to the 8.3 standard except for the special DBOVERLAY files.
3) To keep point 1 valid at all times, support for creating/renaming/removing directories has been disabled.

Thanks for the help GOG.


Imported-from: https://svn.code.sf.net/p/dosbox/code-0/dosbox/trunk@4196

include/dos_system.h

@@ -128,8 +128,8 @@ public:
 	bool UpdateDateTimeFromHost(void);   
 	void FlagReadOnlyMedium(void);
 	void Flush(void);
+	FILE * fhandle; //todo handle this properly
 private:
-	FILE * fhandle;
 	bool read_only_medium;
 	enum { NONE,READ,WRITE } last_action;
 };