Don't persist temporary pool name on devices

Fix a regression accidentally introduced by e0ab3ab.

Additionally, add a new script zpool_import_014_pos.ksh to
the ZFS test suite to exercise 'zpool import -t' functionality.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: loli10K <ezomori.nozomu@gmail.com>
Closes #5466
Closes #5515