hooks: run() return value controls processing of entry data.

Instead of using in-band signaling with the entry_data dict.
I don't know why I didn't think of this in the first place.

hooks: Filter entries by setting entry_data['_hook_cancel'] to True.

The current method only works for plain dicts and other simple mappings.
Mapping that may still contain items after .clear(), like ChainMap, can't
rely on the old method.