Add support for zpool user properties

Usage:

    zpool set org.freebsd:comment="this is my pool" poolname

Tests are based on zfs_set's user property tests.

Also stop truncating property values at MAXNAMELEN, use ZFS_MAXPROPLEN.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Allan Jude <allan@klarasystems.com>
Signed-off-by: Mateusz Piotrowski <mateusz.piotrowski@klarasystems.com>
Sponsored-by: Beckhoff Automation GmbH & Co. KG.
Sponsored-by: Klara Inc.
Closes #11680