OpenZFS 7280 - Allow changing global libzpool variables in zdb and ztest through...

OpenZFS 7280 - Allow changing global libzpool variables in zdb and ztest through command line

Authored by: Pavel Zakharov <pavel.zakharov@delphix.com>
Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Reviewed by: Dan Kimmel <dan.kimmel@delphix.com>
Approved by: Robert Mustacchi <rm@joyent.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Ported-by: George Melikov <mail@gmelikov.ru>
OpenZFS-issue: https://www.illumos.org/issues/7280
OpenZFS-commit: https://github.com/openzfs/openzfs/commit/0e60744
Closes #5676