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()

Makefile.in

@@ -33,6 +33,7 @@ SOURCE=\
 	base/error_state.cc \
 	base/error_string.cc \
 	base/grid_layout.cc \
+	base/file_utils.cc \
 	base/progress_monitor.cc \
 	base/rolling_hash.cc \
 	base/xml_utils.cc \