New module 'stat-size'.

New module 'stat-size'.

* modules/stat-size: New module.  Provides macros for accessing
file size information in instances of struct stat.  Depends on the
fileblocks module because it calls st_blocks.
* lib/stat-size.h: New file, adapted from coreutils' system.h.
* doc/gnulib.texi: Include stat-size.texi.
* doc/stat-size.texi: Documentation for this module.
* m4/stat-size.m4: New file; defines gl_STAT_SIZE.
* m4/fileblocks.m4: Mention that stat-size depends on the call to
AC_STRUCT_ST_BLOCKS.