tests: skip chown/separator with conflicting group IDs

* tests/chown/separator.sh: skip test if the user's group has
multiple entries.
Fixes http://bugs.gnu.org/16532
stat,tail: improve support for HFS+ and HFSX

* src/stat.c (human_fstype): Add new file system ID definitions.
* NEWS: Mention the improvement.
Fixes http://bugs.gnu.org/16336