OpenZFS 9521 - Add checkpoint field

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

Authored by: Eitan Adler <lists@eitanadler.com>
Reviewed by: Serapheim Dimitropoulos <serapheim.dimitro@delphix.com>
Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Reviewed by: kpande <github@tripleback.net>
Reviewed by: Tim Chase <tim@chase2k.com>
Reviewed-by: George Melikov <mail@gmelikov.ru>
Reviewed-by: Giuseppe Di Natale <guss80@gmail.com>
Approved by: Dan McDonald <danmcd@joyent.com>
Ported-by: Brian Behlendorf <behlendorf1@llnl.gov>

OpenZFS-issue: https://illumos.org/issues/9521
OpenZFS-commit: https://github.com/openzfs/openzfs/commit/c5a860f7b
Closes #7658