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: markj
Approved by: markj
Co-authored-by: Mateusz Piotrowski <0mp@FreeBSD.org>
Obtained from: OpenZFS 8eae2d214cfa Add support for zpool user properties
Sponsored by: Beckhoff Automation GmbH & Co. KG.
Sponsored by: Klara Inc.
Differential Revision: https://reviews.freebsd.org/D39657