Want more dynamic kstat tooling

Our kstat framework today has a few shortcomings I bumped into while writing #1033. It would be useful if we had some abstractions which:

• Allow us to attach one or more named kstat fields to an existing (installed) named kstat, assuming no others of the same name are registered.
  • Part of this is down to namespacing: when we create a Port, it takes the name opte$n. This prevents us from defining named stats which XDE cares about on a per-port basis, but OPTE itself does not know about.
• Allow us to make use of the ks_update function pointer.
  • This would allow us to hold e.g., a single source of truth for counters and gauges in #744.
  • This would also allow us to express some disaggregation of stats at runtime: a kstat might be filled out when needed based on sharded state, for instance.
• Make use of other classes of kstat, non-virtual storage, etc. when appropriate.

Start by reading the kstat framework and the context in issues #1033 and #744. Define the required abstractions for attaching named fields, using ks_update, and supporting other kstat storage classes; done means these capabilities are implemented without conflicting registered names and are covered by appropriate tests.