OpenZFS 6872 - zfs libraries should not allow uninitialized variables

Porting notes:

• Many changes were already made in ZoL (for ex. in d4ed66734).

Authored by: Paul Dagnelie <pcd@delphix.com>
Reviewed by: Dan Kimmel <dan.kimmel@delphix.com>
Reviewed by: George Wilson <george.wilson@delphix.com>
Reviewed by: Prakash Surya <prakash.surya@delphix.com>
Reviewed by: Yuri Pankov <yuri.pankov@nexenta.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/6872
OpenZFS-commit: https://github.com/openzfs/openzfs/commit/f83b46b
Closes #5640