Move buffering logic from RageFile into RageFileObj.

Common file interface class, RageBasicFile, shared by RageFile
and RageFileObj.  This makes most uses of these objects interchangeable.
RageFile is now just a simple wrapper for RageFileObj, to create
files in FILEMAN's namespace; file objects can also be created
independently.  This means that, for example, IniFile can be used
to write a file to a CString, without having to jump through hoops,
and without having to use a separate file access wrapper; just do
something like:

RageFileObjMem string_file;
ini.WriteFile( string );
const CString &sString = string_file.GetString();

stepmania/src/RageFileDriverDirect.cpp

@@ -49,7 +49,7 @@ public:
 	virtual int SeekInternal( int offset );
 	virtual RageFileObj *Copy() const;
 	virtual CString GetDisplayPath() const { return path; }
-	virtual int GetFileSize();
+	virtual int GetFileSize() const;
 };
 
 
@@ -365,7 +365,7 @@ int RageFileObjDirect::SeekInternal( int offset )
 	return lseek( fd, offset, SEEK_SET );
 }
 
-int RageFileObjDirect::GetFileSize()
+int RageFileObjDirect::GetFileSize() const
 {
 	const int OldPos = lseek( fd, 0, SEEK_CUR );
 	ASSERT_M( OldPos != -1, strerror(errno) );