As of r364746 (OpenZFS import) existing ZPOOLs are not imported

As of r364746 (OpenZFS import) existing ZPOOLs are not imported
prior to zvol and mountcritlocal resulting in ZVOLs (swap and
virtual machine UFS filesystems) being unavailable, leading to
boot failures.

We move the zpool import from zfs to a new zpool script, with the
-N option to avoid mounting datasets while making the ZPOOL's
datasets available for "legacy" mount (mountpoint=legacy) and ZVOLs
available for subsequent use for swap (in the zvol rc sript) or
for UFS or other filesystems in fstab(5), mounted by mountcritlocal.

Reviewed by:		freqlabs (previous version)
Differential Revision:	https://reviews.freebsd.org/D26185