9521 Add checkpoint field in the default list of the zpool-list man page

illumos/illumos-gate@f41179bd376293096297cdc1f32e610d44f65c8b
https://github.com/illumos/illumos-gate/commit/f41179bd376293096297cdc1f32e610d44f65c8b

https://www.illumos.org/issues/9521

The default list of fields in the zpool list man page is missing checkpoint.

Author: Eitan Adler <lists@eitanadler.com> (eadler)