MFV r316933: 5142 libzfs support raidz root pool (loader project)

illumos/illumos-gate@d5f26ad8122c3762fb16413a17bfb497db86a782
https://github.com/illumos/illumos-gate/commit/d5f26ad8122c3762fb16413a17bfb497db86a782

https://www.illumos.org/issues/5142

the current libzfs only allows simple disk and mirror setup for boot pool, as
loader does support booting from raidz, this feature will remove raidz
restriction from boot pool setup.

FreeBSD note: we have long supported this feature, this commit only
removes a small difference in libzfs.

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>
Author: Toomas Soome <tsoome@me.com>

MFC after: 3 weeks