QtNetwork (examples) - update the second download manager example

It's the one that is more complex - with a text-based 'progress-bar'
and queueing (for some, probably educational, reason) of requests.

Changes:

- update the C++ syntax (mem-initializers, range for, etc.)
- new-style headers
- redirects should not result in creating an empty file.  Since we
  have no UI, and this example is already complex enough, settle
  for just reporting the redirect and removing the empty file.

Task-number: QTBUG-60628
Change-Id: I0b69cd77414ecac7c0bc6b2f8f787befc978de28
Reviewed-by: Edward Welbourne <edward.welbourne@qt.io>