Gittrac #4102:  Changed job_dagman_prio_in_rescue test to check that
a node with a priority set by a parent doesn't get priority written
to rescue DAG; split out HasPriority() and HasExplicitPriority()
so that the new part of job_dagman_prio_in_rescue test works; priority
values are now private with accessor functions so that you can only
change the things you should; more cleanup of "priority adjusting"
code.