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

tests/zfs-tests/include/libtest.shlib

@@ -741,6 +741,18 @@ function bkmarkexists
 	return $?
 }
 
+#
+# Return 0 if a hold exists; $? otherwise
+#
+# $1 - hold tag
+# $2 - snapshot name
+#
+function holdexists
+{
+	zfs holds "$2" | awk '{ print $2 }' | grep "$1" > /dev/null 2>&1
+	return $?
+}
+
 #
 # Set a property to a certain value on a dataset.
 # Sets a property of the dataset to the value as passed in.