MFC r251634: illumos #3745 zpool create should treat -O mountpoint and -m the same

MFC r251634: illumos #3745 zpool create should treat -O mountpoint and -m the same

cddl/contrib/opensolaris/cmd/zpool/zpool_main.c:  (change 644608)
	This allows specifying a mountpoint using the latter form and having
	its value checked and used as it would be using the former form.

	As a consequence of this change:

	1. The mountpoint property is set in the fsprops nvlist prior
	   to creating the pool, rather than being set after creating
	   the pool.  To me, this is the proper approach, since it
	   avoids creating the pool if the mountpoint setting would
	   cause the command to fail.
	2. The mountpoint property, unlike all others, can be specified
	   more than once.  Only the last setting takes effect.  This
	   is to avoid breaking potential existing users that specify
	   -m more than once.

Submitted by:	will

cddl/contrib/opensolaris/lib/libzfs/common/libzfs_pool.c
	Fix "zpool create -R <whatever> -m <whatever>".  Ever since
	change 644608, this has been broken.  The problem is that some
	old code in libzfs_pool.c would force a pool's mountpoint to
	"/" when creating a pool with an altroot.  That probably
	implemented some old policy decision regarding altroots, but it
	conflicts with the current manpage.  It also had no effect
	until 644608, because the zpool command would _always_ change
	the pool's mountpoint after creating it.  The solution is to
	delete the old code from libzfs_pool.c.

Submitted by:	asomers

Reviewed by:	Matthew Ahrens <mahrens@delphix.com>,
		Christopher Siden <christopher.siden@delphix.com>
Sponsored by:	Spectra Logic

git-svn-id: svn://svn.freebsd.org/base/stable/9@252758 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f