Fix log truncating in DAGMan #3591

Description:
   DAGMan doesn't truncate the new "workflow" or "default" log file on
(non-recovery) startup.  This can lead to serious errors.

   Here's how to reproduce at least one instance of a problem:
   1: add a sleep to job_dagman_prepost-prehello.pl (so you can hold the
		DAG while PRE scripts are running).
   1: Run job_dagman_prepost.dag, and let it go to completion.
   1: Run job_dagman_prepost.dag again.
   1: While the PRE scripts are running, do condor_hold/condor_release on the DAGMan job.

When DAGMan is released, it will re-read the log from the first run,
and immediately decide that the DAG is finished, even if the PRE scripts
are still running!
Fix for #3591

We watch the default log from the outset, not after the prescripts start
running.
Rename lock file after we sleep #3591