allow ZFS pool to have temporary name for duration of current import

The change adds -t <name> option to zpool create and -t option to zpool
import in its form with an old name and a new name. This allows to
import (or create) a pool under a name that's different from its real,
permanent name without affecting that name. This is useful when working
with VM images or images of other physical systems if they happen to
have a ZFS pool with the same name as the host system.

The changes come from ZoL with some small tweaks.
The porting has been done by julian.

The change is being submitted to OpenZFS:
https://github.com/openzfs/openzfs/pull/600

Submitted by: julian
Reviewed by: smh
MFC after: 2 weeks
Sponsored by: Panzura (porting)
Differential Revision: https://reviews.freebsd.org/D14972