Long opts for zdb

This change introduces long options for zdb. It updates the usage
message as well to include the long options.

Reviewed-by: Matthew Ahrens <mahrens@delphix.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Manoj Joseph <manoj.joseph@delphix.com>
Closes #12818