Prevent task prolog buffer overflow

Prevent a possible buffer overflow if the task prolog output
exceeds 4096 bytes.
remove vestigial/unused variables
Change some scheduling logs and variables

Change some sched/backfill messages to be more clear
Change some sched/builtin messages and variable names to
no longer reference "backfill".
Improve documentation for scheduling configuration
Minor refinements to main scheduling loop
reduce scheduler verbosity
Scheduling tweak

Partial reversion of commit 9b1dadea4eb823b5ef29d8b4ee56cb6b7c3be22f
Merge branch 'slurm-14.03'