OpenZFS 5142 - libzfs support raidz root pool (loader project)

Authored by: Toomas Soome <tsoome@me.com>
Reviewed by: - George Wilson <george.wilson@delphix.com>
Reviewed by: - Yuri Pankov <yuri.pankov@nexenta.com>
Reviewed by: - Andrew Stormont <andyjstormont@gmail.com>
Reviewed by: - Albert Lee <trisk@omniti.com>
Approved by: - Robert Mustacchi <rm@joyent.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Giuseppe Di Natale <dinatale2@llnl.gov>
Ported-by: George Melikov <mail@gmelikov.ru>

OpenZFS-issue: https://www.illumos.org/issues/5142
OpenZFS-commit: https://github.com/openzfs/openzfs/commit/d5f26ad
Closes #5663