WIP illumos commit:

commit 2acef22db7808606888f8f92715629ff3ba555b9
Author: Matthew Ahrens <mahrens@delphix.com>
Date:   Tue Oct 8 09:13:05 2013 -0800

    4171 clean up spa_feature_*() interfaces
    4172 implement extensible_dataset feature for use by other zpool features
    Reviewed by: Max Grossman <max.grossman@delphix.com>
    Reviewed by: Christopher Siden <christopher.siden@delphix.com>
    Reviewed by: George Wilson <george.wilson@delphix.com>
    Reviewed by: Jerry Jelinek <jerry.jelinek@joyent.com>
    Approved by: Garrett D'Amore <garrett@damore.org>