Initial attempt to document events (subclasses) and their payload.
In no way complete - most have been trial and error and some deducing
what they could mean. It needs more information, from someone that
knows the code better. But it's a start...
Create a zpool.export event (with corresponding script)
that's triggered when a pool is exported.
Create a zpool.import event (with corresponding script)
that's triggered when a pool is imported.