Support recursing on datasets about zam HOT 2 OPEN

blocklist can be implemented using ZFS User Properties: https://docs.oracle.com/cd/E19120-01/open.solaris/817-2271/gdrcw/index.html
Have a zam:include property that takes values {yes,no}? IIRC ZFS will let you choose whether or not a property value is inherited by descendent datasets.
It would probably be easiest to change the top level config to only be defined on a zpool, then use user properties to define which specific datasets to include/exclude. However, I still want the recursive ZAM config so that I can, for example, define a default backup location for a pool, then for a specific dataset define multiple backup locations.

from zam.

Tentative plan: managed_dataset_t will be renamed to ~group_t and define a name instead of a source. Then zam will get the zam:group property of all data sets, and match it against the group names from the config. Each dataset can only be in one group.

from zam.