Added attribute rb_file along rb_path and rb_name.

This is useful only for reading back a single target device having
a simple path without a target id folder in between.
Fixed reading back files of file_tag.

One constant of data_tag was overlapping with one of file_tag.
Fixed file writing by not removing it first.

Files are overwritten by default until O_APPEND is specified.
Removing files first could remove a symbolic link resulting into
a wrong file location.
Added missing flag O_TRUNC.
Fixed O_TRUNC to not truncate a symbolic link.

This is done by invoking readlink first.
If it is a link, we invoke openat(link_dir, link, ...).
If it is a file, we invoke openat(AT_FDCWD, file, ...).
Using realpath instead of readlink.
Fixed file creation.
Updated daq_tag.