OpenZFS 5704 - libzfs can only handle 255 file descriptors

Authored by: Simon Klinkert <simon.klinkert@gmail.com>
Reviewed by: Josef 'Jeff' Sipek <josef.sipek@nexenta.com>
Reviewed by: John Kennedy <john.kennedy@delphix.com>
Approved by: Richard Lowe <richlowe@richlowe.net>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed by: Ned Bass <bass6@llnl.gov>
Reviewed-by: Chunwei Chen <david.chen@osnexus.com>
Ported-by: George Melikov <mail@gmelikov.ru>

OpenZFS-issue: https://www.illumos.org/issues/5704
OpenZFS-commit: https://github.com/openzfs/openzfs/commit/bde3d61
Closes #5767