Misc interrupts/LWKT 1/2: threaded interrupts 2: Major work on the

Misc interrupts/LWKT 1/2: threaded interrupts 2: Major work on the
user scheduler, separate it completely from the LWKT scheduler and make
user priorities, including idprio, normal, and rtprio, work properly.
This includes fixing the priority inversion problem that 4.x had.
Also complete the work on interrupt preemption.  There were a few things
I wasn't doing correctly including not protecting the initial call
to cpu_heavy_restore when a process is just starting up.  Enhance DDB a
bit (threads don't show up in PS yet).

This is a major milestone.