Illumos #3745, #3811

3745 zpool create should treat -O mountpoint and -m the same
3811 zpool create -o altroot=/xyz -O mountpoint=/mnt ignores

the mountpoint option

Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Approved by: Christopher Siden <christopher.siden@delphix.com>

References:

https://www.illumos.org/issues/3745
https://www.illumos.org/issues/3811
illumos/illumos-gate@8b713775314bbbf24edd503b4869342d8711ce95

Ported-by: Richard Yao <ryao@gentoo.org>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Issue #1775