If a job is submitted only with -c option and numcpus is updated before

the job starts update the cpus_per_task appropriately.

This also moves update num_tasks to after the setting of node counts on
an update.  It didn't appear to matter, but the cpus_per_task and
pn_min_cpus had to be figured out after the cpus and nodes were set but
before tasks.

Bug 2302