Introduce getting holds and listing bookmarks through ZCP

Introduce getting holds and listing bookmarks through ZCP

Consumers of ZFS Channel Programs can now list bookmarks,
and get holds from datasets. A minor-refactoring was also
applied to distinguish between user and system properties
in ZCP.

Reviewed-by: Paul Dagnelie <pcd@delphix.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Matt Ahrens <mahrens@delphix.com>
Reviewed-by: Serapheim Dimitropoulos <serapheim@delphix.com>
Ported-by: Serapheim Dimitropoulos <serapheim@delphix.com>
Signed-off-by: Dan Kimmel <dan.kimmel@delphix.com>
OpenZFS-issue: https://illumos.org/issues/8862
Closes #7902