allow multiple job-manager prolog commands

[grondo]

Currently the perilog.so job manager plugin only allows a single job-manager.prolog and epilog command. Since most sites have a system prolog (which will execute per-job-rank typically), this doesn't allow other components to easily schedule a script to run before job shells are started.

There should be some way to configure an arbitrary number of other scripts that run as prolog (or epilog). To avoid breaking new configs, perhaps new [[job-manager.prologs]] and [[job-manager.epilogs]] arrays with the same supported keys as the job-manager.prolog and epilog table, perhaps with the addition of a name field for use in the description emitted in the prolog/epilog-start and finish events.