handle spaces in dataset names

Firstly, let me say thanks for this very useful tool, I've used it on a bunch of machines, and it's made snapshot maintenance a breeze!

The only problem I've had is that one of my filesystems has datasets with spaces in their names, so I had to modify a couple files to add quotes and split on tabs:

* where `system` is called, make sure dataset/snapshot names are quoted
* where zfs list is called, make sure the output is split on tabs since
  we use the -H flag which forces tabs as delimiters to the fields

-Caleb