This prevents rebuilding a job's dependency string when it has at least
one invalid (never satisfied) dependency, no matter if such invalid
dependency has already been purged (after MinJobAge) or not. This can
be useful to track down the culprit invalid dependencies even after they
are gone from ctld's in-memory job list.

The flag is cleared upon a successful job dependency update or after
another job in the dependency list has been satisfied if such list is
composed with the '?' symbol (OR'ed).

Bug 5851.