Spin-off syscall-related file operations (#78)

* [file_utils] spin-off syscall-related file operations

1. Eliminate the potential circular dependency between
   persistent-data/block.h and persistent-data/file_utils.h,
   if the former one wants to include the latter.
2. Avoid namespace pollution by removing the "using namespace std"
   declaration in block.tcc.
3. Correct the header hierarchy: base/xml_utils.h now no longer
   depends on the higher-level persistent-data/file_utils.h

* [file_utils] support block files in get_file_length()

base/xml_utils.h

@@ -36,7 +36,6 @@ namespace xml_utils {
 		void parse(std::string const &backup_file, bool quiet);
 
 	private:
-		size_t get_file_length(string const &file) const;
 		unique_ptr<base::progress_monitor> create_monitor(bool quiet);
 
 		XML_Parser parser_;