stat: report the correct block size for file system usage

stat: report the correct block size for file system usage

struct statfs has the f_frsize member since Linux 2.6,
so use that rather than f_bsize which can be different.
Note the related df change mentioned in NEWS is handled
in gnulib by using statvfs() rather than statfs()
on Linux > 2.6.36 (where statvfs doesn't hang) and the
same method as stat for Linux 2.6 kernels earlier than that.
stat(1) doesn't use statvfs() on GNU/Linux as the f_type
member isn't available there.
Note the change to not use statvfs() on GNU/Linux was introduced
in gnulib commit eda39b8 16-08-2003.

* m4/stat-prog.m4 (cu_PREREQ_STAT_PROG): Check for the f_frsize
member in the statfs structure.
* src/stat.c: Use (struct statfs).f_frsize if available.
* NEWS (Bug fixes): Mention this stat fix, and the related df fix
coming in the next gnulib update.
* THANKS.in: Add Nikolaus.

Reported and Tested by Nikolaus Rath