devel/py-libzfs: Fix build on 12 and main

FreeBSD 12 doesn't have userobj accounting or project quotas. OpenZFS
master has a new flags parameter to many zfs_iter_* functions.

Add checks for SPA_FEATURE_* enum values newer than what's in 12.3 and
use this info to help generate zfs_userquota_prop_t and spa_feature_t
appropriately.

Add checks for the number of parameters to various zfs_iter_* functions
and wrapper functions ZFS._iterate* to eat the flags.

Approved by: lwhsu (implicit)
Sponsored by: iXsystems, Inc.