Make formatting of dedup values string consistent

All other prop values return options separated by | ,
dedup values do not, they are separated by , . This change
makes the dedup value formatting consistent with other properties.

Reviewed-by: George Melikov <mail@gmelikov.ru>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Ryan Moeller <ryan@iXsystems.com>
Signed-off-by: Clint Armstrong <clint@clintarmstrong.net>
Closes #10761