sched/sched: Implements a global scheduler lock capability as part of SMP...

sched/sched:  Implements a global scheduler lock capability as part of SMP support.  This allows the scheduler to be locked with no knowledge or access to the TCB of the currently running task.  This is necessary because accessing the TCB of the currenlty running task is, itself, a non-atomic operation.  This global scheduler lock cpability was add just to support that atomic access to the TCB.