Check background requests are allowed before starting

Check background requests are allowed before starting

If background requests are not allowed, don't even attempt to start
the network session, just fail the request immediately.

After this change, a QNAM with mixed requests queued should behave
as follows when background requests are disabled:
 - background requests at head of the queue fail
 - first foreground request starts the session and succeeds
 - remaining background requests fail
 - remaining foreground requests succeed

If policy is changed on the fly, then running background requests
are not aborted. However queued background requests won't be started.

Change-Id: Ic9aba1eb59ca41b166a08d2ed09418e1b6af6b60
Reviewed-by: Lars Knoll <lars.knoll@nokia.com>
Reviewed-by: Martin Petersson <Martin.Petersson@nokia.com>