zpool/zfs commands that fail as non-root user should return a friendly error

[dch]

Via https://openzfsonosx.org/forum/viewtopic.php?f=24&t=492

When I ran "zpool import" I got back "no pools available".

That was after a panic so I thought the sh*t had hit the fan again and mentally prepared to re-load 3 TB data again. But fortunately, I saw that I ran as myself, not as root. After a sudo -s everything looked as it should.

If one run zpool import as a normal user, can you please add some warning or whatever. Maybe if one cannot access any /dev/* for writing or something. As it is now it is a bit too subtle.

[ilovezfs]

As a start 72d8832

This is close to a duplicate of #74