- fix a (now-) bug in kernel/softirq.c, it did a wakeup outside any
   atomic regions, which falsely identified random processes as a
   non-atomic wakeup, and which causes random priority boost to be
   distributed.

 - reset the initial idle thread's priority back to PRIO_MAX after doing
   the wakeup_forked_process() - correct preemption relies on this.

 - update current->prio immediately after a backboost.

 - clean up effective_prio() & sleep_avg calculations so that there are
   fewer RT-task special cases.  This has the advantage of the sleep_avg
   being maintained even for RT tasks - this could be advantegous for
   tasks that briefly enter/exit RT mode.