use SYSTEM_PERIODIC_HOLD_NAMES to improve UX in the face of held jobs

One of the most frequent issues users encounter in a Condor pool, and one of the most difficult for them to understand and debug, is held jobs. One important incremental improvement to the UX is breaking the multi-part SYSTEM_PERIODIC_HOLD expression into sub-expressions via SYSTEM_PERIODIC_HOLD_NAMESso that Condor can tell the user (and/or admin) which component of the expression "fired" and needs to be addressed.