utimens: check for ctime update

utimens: check for ctime update

futimens/utimensat on Linux fails to bump ctime if mtime is
UTIME_OMIT and atime is specified.

* tests/test-utimens-common.h (check_ctime): Define.
* tests/test-utimens.h (test_utimens): Expose the Linux bug.
* tests/test-futimens.h (test_futimens): Likewise.
* tests/test-lutimens.h (test_lutimens): Likewise.
* doc/posix-functions/futimens.texi (futimens): Document the bug.
* doc/posix-functions/utimensat.texi (utimensat): Likewise.

Signed-off-by: Eric Blake <ebb9@byu.net>