score: Add thread priority to scheduler nodes
The thread priority is manifest in two independent areas. One area is
the user visible thread priority along with a potential thread queue.
The other is the scheduler. Currently, a thread priority update via
_Thread_Change_priority() first updates the user visble thread priority
and the thread queue, then the scheduler is notified if necessary. The
priority is passed to the scheduler via a local variable. A generation
counter ensures that the scheduler discards out-of-date priorities.
This use of a local variable ties the update in these two areas close
together. For later enhancements and the OMIP locking protocol
implementation we need more flexibility. Add a thread priority
information block to Scheduler_Node and synchronize priority value
updates via a sequence lock on SMP configurations.
Update #2556.