Abort background requests if policy changes to disallow them

Abort background requests if policy changes to disallow them

Using the policy change signal from QNetworkSession.
If the new policy disallows background requests and this is a
background request, then generate an error.
This results in a TCP RST on the socket, and a
BackgroundRequestNotAllowedError on the QNetworkReply.

If the reply is already finished, no action is taken.

Change-Id: I4ff5c681a8b7b852727bb95f03664d666f4efe07
Reviewed-by: Martin Petersson <Martin.Petersson@nokia.com>