Deprecate dedupditto

This documents, in zpool.8, that dedupditto is deprecated and will be
made to have no effect in a future release.

Reviewed-by: George Melikov <mail@gmelikov.ru>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Matthew Ahrens <mahrens@delphix.com>
Signed-off-by: Richard Laager <rlaager@wiktel.com>
Closes #8650