kernel - usched_dfly revamp (4), improve tail

kernel - usched_dfly revamp (4), improve tail

* Improve tail performance (many more cpu-bound processes than available
  cpus).

* Experiment with removing the LWKT priority adjustments for kernel vs user.
  Instead give LWKT a hint about the user scheduler when scheduling a thread.
  LWKT's round-robin is left unhinted to hopefully round-robin starved LWKTs
  running in kernel mode.

* Implement a better calculation for the per-thread uload than the priority.
  Instead, use estcpu.

* Adjust default weigntings for new uload calculation scale.