msgport.9: lwkt_initport_spin now takes a fixed_cpuid argument.
kernel - minor cpu idle statistics adjustment

* Change the idlethread test from RQF_AST_LWKT_RESCHED to
  RQF_IDLECHECK_WK_MASK (which includes the first flag and adds a few more)
  to determine if the idle thread is actually idle or not.

* Should not materially change reported idle% as the original test handled
  the most common idle-thread-skips-halt case.