The stop-on-error option aims to solve the lack of damage_visitor support
in space_map::count_metadata and btree counting_visitor. To prevent
possibly silent errors during space map or btree counting, the option
stops the counting process immediately when a btree or bitmap error
was found.
The bitmap blocks are also validated to avoid potential false-alarm,
if broken bitmap is not counted, and the corresponding ref-count is
also zeroed.
* [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()
