MFC r316858 7280 Allow changing global libzpool variables in zdb

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

illumos/illumos-gate@0e60744c982adecd0a1f146f5637475d07ab1069
https://github.com/illumos/illumos-gate/commit/0e60744c982adecd0a1f146f5637475d07ab1069

https://www.illumos.org/issues/7280

zdb is very handy for diagnosing problems with a pool in a safe and
quick way. When a pool is in a bad shape, we often want to disable some
fail-safes, or adjust some tunables in order to open them. In the
kernel, this is done by changing public variables in mdb. The goal of
this feature is to add the same capability to zdb and ztest, so that
they can change libzpool tuneables from the command line.

Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Reviewed by: Dan Kimmel <dan.kimmel@delphix.com>
Approved by: Robert Mustacchi <rm@joyent.com>
Author: Pavel Zakharov <pavel.zakharov@delphix.com>

MFC after: 3 weeks