Use string constants for dataset types about go-zfs

[bakins]

We'd have to change how we parse the ZFS output to do this. Let me mock up a PR for just that.

[mdlayher]

Sure thing! No rush, just want to eliminate some common errors. I've seen it happen too many times where one would check for something like a snpashot or colne, you know?

[bakins]

See #17 - quick hack done during a meeting.

[mdlayher]

I just realized that "clone" is not a valid dataset type, since a clone is actually just a filesystem. I won't be adding clone then.

[mdlayher]

Closed by #19.