MFV r302658: 6872 zfs libraries should not allow uninitialized variables

MFV r302658: 6872 zfs libraries should not allow uninitialized variables

illumos/illumos-gate@f83b46baf98d276f5f84fa84c8b461f412ac1f5e
https://github.com/illumos/illumos-gate/commit/f83b46baf98d276f5f84fa84c8b461f41
2ac1f5e

https://www.illumos.org/issues/6872
  We compile the zfs libraries with -Wno-uninitialized. We should remove
  this. Change makefiles, fix new warnings, fix pbchk errors.

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>
Author: Paul Dagnelie <pcd@delphix.com>