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

5134 if ZFS_DEBUG or debug= is set, libzpool should enable debug prints
Reviewed by: Adam Leventhal <adam.leventhal@delphix.com>
Reviewed by: Christopher Siden <christopher.siden@delphix.com>
Reviewed by: George Wilson <george.wilson@delphix.com>
Reviewed by: Saso Kiselkov <skiselkov.ml@gmail.com>
Approved by: Dan McDonald <danmcd@omniti.com>

References:

https://www.illumos.org/projects/illumos-gate/issues/5134
https://github.com/illumos/illumos-gate/commit/7fa49ea

Porting notes:

Added dprintf_setup() to main in zfs_main.c and zpool_main.c.

Ported by: Turbo Fredriksson <turbo@bayour.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #2669