zdb: detect cachefile automatically otherwise force import

If a pool is created with the cache file located in a non-default
path /etc/default/zpool.cache, removed, or the cachefile property
is set to none, zdb fails to show the pool unless we specify the
cache file or use the -e option. This PR automates this process.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Reviewed-by: Akash B <akash-b@hpe.com>
Signed-off-by: Ameer Hamza <ahamza@ixsystems.com>
Closes #16071