Chainable host-exclude entries

dlofthouse had a good suggestion the other day to ease development of new extensions – allow chaining of host-exclude content.

Basically this means internally keep a graph of the various kernel management API versions that are the id for a version for which we support mixed domains. (Graph is currently a chain although in theory it could be a tree.) Then in domain.xml it's only necessary to add a given extension to the host-exclude element for the latest version in which it doesn't appear. The system then knows to exclude it from all earlier versions without requiring us spelling it out in domain.xml.