Json

Note: not necessarily indending to push this to zfsonlinux.  Creating a pull request seems to be the only way of leaving code review comments?  Let me know if I'm using github wrong.
I think you should use zfs_prop_to_name().  I don't think that this code (column_name()) will generate the output you have in your example (e.g. it will use "REFER" instead of "referenced").