Illumos 5134 - if ZFS_DEBUG or debug= is set, libzpool should enable debug prints

Reviewed by: Christopher Siden christopher.siden@delphix.com
Reviewed by: Adam Leventhal adam.leventhal@delphix.com
Original author: Matthew Ahrens

References:
  https://www.illumos.org/projects/illumos-gate/issues/5134
  https://reviews.csiden.org/r/82/

Ported by: Turbo Fredriksson <turbo@bayour.com>