This brings in the new fopen-gnu module and the RF_SENSITIVE flag for
fread_file and read_file.  This also adds the following changes to be
consistent with the latest changes in Gnulib:
- the callers of fread_file and read_file to be adjusted for the FLAGS
  argument
- "attribute.h" needs to be used extensively

Signed-off-by: Daiki Ueno <ueno@gnu.org>