stat: use e.g. %.3X instead of %X.%3:X for sub-second precision

stat: use e.g. %.3X instead of %X.%3:X for sub-second precision

* NEWS: Document this.
* doc/coreutils.texi (stat invocation): Likewise.
* gl/lib/fstimeprec.c, gl/lib/fstimeprec.h, gl/modules/fstimeprec:
* gl/modules/fstimeprec-tests, gl/tests/test-fstimeprec.c:
New files.
* bootstrap.conf (gnulib_modules): Add fstimeprec.
* src/stat.c: Include fstimeprec.h.  Don't include xstrtol.h.
(decimal_point, decimal_point_len): New static vars.
(main): Initialize them.
(epoch_sec, out_ns): Remove.
(out_int, out_uint): Now returns whatever printf returned.
(out_minus_zero, out_epoch_secs): New functions.
(print_stat): Use out_epoch_sec instead of out_ns and epoch_sec.
(print_stat, print_it, usage): Remove the %:X-style formats.
* tests/misc/stat-nanoseconds: Set TZ=UTC0 to avoid problems
with weird time zones.  Use a time stamp near the Epoch so that we
don't have to worry about leap seconds.  Redo test cases to match
new behavior.
* tests/touch/60-seconds: Change %Y.%:Y to %.9Y, to adjust to
new behavior.