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()

persistent-data/block_counter.h

@@ -38,7 +38,7 @@ namespace persistent_data {
 		virtual void inc(block_address b) {
 			count_map::iterator it = counts_.find(b);
 			if (it == counts_.end())
-				counts_.insert(make_pair(b, 1));
+				counts_.insert(std::make_pair(b, 1));
 			else
 				it->second++;
 		}