Signed-off-by: NEli Atzaba <eliat123@gmail.com>

Websites that return a wrong ``Content-Length`` header may cause a data
loss error. Also when a chunked response is not finished properly.

This change adds a new setting ``DOWNLOAD_FAIL_ON_DATALOSS`` (default:
``True``) and request.meta key ``download_fail_on_dataloss``.

Avoid loading settings from environment in scrapy core.
Instead it's better to populate them from the starting
shell or an embedding script.

Fix documentation for S3_STORE_ACL (now settings.FILES_STORE_S3_ACL) settings: it has nothing to do with feed exporters.

Fixes #1593

For backwards compatibility reasons the default value is not changed.

* `REDIRECT_MAX_METAREFRESH_DELAY` has been deprecated and was
  renamed to `METAREFRESH_MAXDELAY`.
* Merge duplicate documents about `METAREFRESH_MAXDELAY` appeared both
  in the settings page and the downloader-middlewares page.