score: Replace _Scheduler_Allocate/Free()

Replace _Scheduler_Allocate() with _Scheduler_Node_initialize().  Remove
the return status and thus the node initialization must be always
successful.

Rename _Scheduler_Free() to _Scheduler_Node_destroy().
score: Rename _Scheduler_Update()

Rename _Scheduler_Update() to _Scheduler_Update_priority().  Add
parameter for the new thread priority to avoid direct usage of
Thread_Control::current_priority in the scheduler operation.