Add zfeature_checks_disable module option

Tuning setting to disable all feature checks while opening pools, allowing pools with unsupported features to be opened.

There's been a few discussion on how to import pools created on other operating system, and most have stumbled on a feature that isn't available on Linux.

Since the code and variable is already there, having it being turned on or of without recompiling the code might be a good thing...

Signed-off-by: Turbo Fredriksson <turbo@bayour.com>