rem: void extract_single_file(const entry& dict, file_entry& target)
add: void extract_single_file(const entry& dict, file_entry& target , std::string const& root_dir)
context: 	void extract_single_file(const entry& dict, file_entry& target)	{		target.size = dict["length"].integer();		const entry::list_type& list = dict["path"].list();		for (entry::list_type::const_iterator i = list.begin();			i != list.end();			++i)		{			target.path /= i->string();		}		if (target.path.is_complete()) throw std::runtime_error("torrent contains "			"a file with an absolute path: '"			+ target.path.native_file_string() + "'");	}