Illumos #3098

3098 zfs userspace/groupspace fail without saying why when run as non-root

Reviewed by: Eric Schrock <eric.schrock@delphix.com>
Approved by: Richard Lowe <richlowe@richlowe.net>

Ported-by: Tim Chase <tim@chase2k.com>

This was ported as a prerequisite to the forthcoming Illumos 3464 patch.