sched/pthread and task: When a pthread is started, there is a small bit of...

sched/pthread and task:  When a pthread is started, there is a small bit of logic that will run on the thread of execution of the new pthread.  In the case where the new pthread has a lower priority than the parent thread, then this could cause both the parent thread and the new pthread to be blocked at the priority of the lower priority pthread (assuming that CONFIG_PRIORITY_INHERITANCE is not selected).

This change temporarily boosts the priority of the new pthread to at least the priority of the new pthread to at least the priority of the parent thread.  When that bit of logic has executed on the thread of execution of the new pthread, it will then drop to the correct priority (if necessary) before calling into the new pthread's entry point.