Illumos #2762: zpool command should have better support for feature flags

2762 zpool command should have better support for feature flags
Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Reviewed by: George Wilson <george.wilson@delphix.com>
Approved by: Eric Schrock <Eric.Schrock@delphix.com>

References:

illumos/illumos-gate@57221772c3fc05faba04bf48ddff45abf2bbf2bd
https://www.illumos.org/issues/2762

Ported-by: Brian Behlendorf <behlendorf1@llnl.gov>