hammer2 - Retool dmsg mechanics to improve virtual circuit design 2/2

hammer2 - Retool dmsg mechanics to improve virtual circuit design 2/2

* Use transaction stacking to effectively connect resources to consumers.
  Issuing a transaction over a received SPAN effectively creates a virtual
  circuit to the originator.

* The hammer2 service demon (userland) is currently being used to route,
  but direct kernel<->kernel socket connections are now theoretically
  possible for later performance work.  Current performance is going
  to be low due to layering.

* Get subr_diskiocom and the xdisk driver mostly working again.  The
  code is a lot cleaner but still needs considerable stability work and
  better unwinding during failure conditions.

* Start work on hammer2<->hammer2 communications.