6356: crunch-job doesn't create new tasks after job success is set.

6356: crunch-job doesn't create new tasks after job success is set.

#6356 reported that a permanently failed task was retried.  Note 3
discusses why this happened and suggests two fixes:

* Only put tempfailed task back on the todo list.
* Run `last THISROUND if $main::please_freeze || defined($main::success);`
  after we call reapchildren(), since it's the main place where the
  value of $main::success can change.

The first change would revert part of
75be7487c2bbd83aa5116aa5f8ade5ddf31501da, which intentionally puts
these tasks back on the todo list to get a correct tasks count.

The current `last if…` line was added in
b306eb48ab12676ffb365ede8197e4f2d7e92011, with the rationale "Don't
create new tasks if $main::success is defined."  This change corrects
the code to implement the desired functionality, by checking and
stopping just before we create a new task (functionally, at least).