tests: make ls tests independent of COLORTERM env

Since the recent commit v8.22-68-g08783f1, ls coloring
is now dependent on the COLORTERM environment variable.

* tests/envvar-check: Unset COLORTERM from test environment.
* tests/ls/color-dtype-dir.sh: Ensure coloring is used.
* tests/misc/ls-misc.pl: Likewise.

Prompted by the continuous integration build failure at:
http://hydra.nixos.org/build/10397646
doc: clarify meaning of stat's mtime and ctime formats

* src/stat.c (usage): s/modification time/data modification time/;
s/change time/status change time/
* doc/coreutils.texi: Ditto.
maint: remove trailing comma from enum to make gl/ c89 compat

* gl/lib/fadvise.c (fadvice_t): This might go to gnulib some day so
remove the trailing comma.
Fixes http://bugs.gnu.org/17329
maint: make ChangeLog generation more robust

* Makefile.am (gen-ChangeLog): Sync changes from GNU hello,
to ensure exit status is propagated, and to support an optional
git-log-fix file.