Enhanced zpool binary lookup to try common paths when exec.LookPath fails.
This fixes issue #718 where TrueNAS SCALE reports inflated storage because
the agent runs with a restricted PATH that doesn't include /usr/sbin.
Changes:
- Added findZpool() helper that tries common paths like /usr/sbin/zpool,
/sbin/zpool, /usr/local/sbin/zpool for TrueNAS/FreeBSD/Linux systems
- Added commonZpoolPaths variable listing typical zpool locations
- Added tests for the new findZpool function
This ensures zpool list is used for accurate pool-level capacity instead
of falling back to dataset-level summation.
