ztest: propagate -o to the zdb child process

I think this is the behavior that most users expect.

Future work: have a separate flag, e.g., -O, to specify separate
set_global_vars for the zdb child than for the ztest children.

Reviewed-by: Matthew Ahrens <mahrens@delphix.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Pavel Zakharov <pavel.zakharov@delphix.com>
Signed-off-by: Christian Schwarz <me@cschwarz.com>
Closes #11602