Allow a network configuration to be included in a proxy query

Allow a network configuration to be included in a proxy query

When Qt is compiled with bearer management support, the network
configuration can be included as a parameter in QNetworkProxyQuery.

This allows QNetworkProxyFactory::systemProxyForQuery to get the right
proxy setting for a specific network. For example a mobile phone could
have network configurations for home WLAN, work WLAN and 3G data
access points, each with different proxy configurations.

Task-number: QTBUG-18618
Reviewed-by: Peter Hartmann