Fix memory corruption during parallel zpool import with -o cachefile (#16419)

When importing multiple pools, the nvlist of properties given with "-o"
is shared amongst the several threads. So no thread should modify it.
Previously, in the course of validating the cachefile property, the
zpool_valid_proplist function would temporarily modify the value, and
then change it back. Now it will operate on a clone of the value.

Sponsored by: Axcient
Fixes #16405
Signed-off-by: Alan Somers <asomers@gmail.com>
Reviewed-by: Tony Hutter <hutter2@llnl.gov>
Reviewed-by: Allan Jude <allan@klarasystems.com>
Reviewed-by: George Wilson <gwilson@delphix.com>
Reviewed-by: Alexander Motin <mav@FreeBSD.org>