hammer2 - flush sequencing part 2

* API adjustments.  Add trans to setsubmod and pass &chain to
  hammer2_chain_modify() so it can (soon) modify/duplicate and
  return a different chain.

* Tentitively add flush_tid tracking to the hmp which will
  soon be used by the modify code to determine when a duplication
  is required.
Merge branch 'master' of ssh://crater.dragonflybsd.org/repository/git/dragonfly
cpdup - add -n (not-for-real) option

* Add cpdup -n, which causes cpdup to go through the motions but not
  actually make any modifications.
ahci(4): Maximum I/O size is 2MB