build: update gnulib submodule to latest
cp: copy files by inode only if that facility is available

* src/copy.c (copy_dir): Use the new SAVEDIR_SORT_FASTREAD, not
SAVEDIR_SORT_INODE.  Problem reported by Bernhard Voelker in:
http://lists.gnu.org/archive/html/coreutils/2014-02/msg00037.html
date: fix crash or infinite loop when parsing a malformed TZ=""

* NEWS: Mention the fix.
* gnulib: Update to incorporate the fix.
This is the only change in this gnulib update.
* tests/misc/date.pl: Add a test for this case.

Fixes http://bugs.gnu.org/16872
doc: improve df --human and --si, help and man page

* src/df.c (usage): Adjust the --human and --si descriptions
to not depend on each other.  Also include an example that is
illustrative of the rounding, suffix, width, and localized fractions.
* src/system.h (emit_size_note). Adjust so that it's obvious the
description is pertaining to the input SIZE argument, and not
to any sizes that might be output by df for example.
Fixes http://bugs.gnu.org/16922