5704 libzfs can only handle 255 file descriptors

illumos/illumos-gate@bde3d612a7c090234c60e6e4578821237a5db135
https://github.com/illumos/illumos-gate/commit/bde3d612a7c090234c60e6e4578821237a5db135

https://www.illumos.org/issues/5704

libzfs uses fopen(), at least in libzfs_init(). If there are more than 255
filedescriptors open, fopen() will fail unless you give 'F' as the last mode
character. The fix would be to give 'rF' instead of 'r' as mode to fopen().

Reviewed by: Josef 'Jeff' Sipek <josef.sipek@nexenta.com>
Reviewed by: John Kennedy <john.kennedy@delphix.com>
Approved by: Richard Lowe <richlowe@richlowe.net>
Author: Simon Klinkert <simon.klinkert@gmail.com>