Fix a number of bugs with windows system proxies

Fix a number of bugs with windows system proxies

TcpServer requests always returned no proxy, even if socks was available
Tag handling was broken for empty tag (if system proxies were tagged)
Tag handling was broken for unknown tags - now handled the same as if
    no tag was given at all.
When there are different proxies for http and https, windows returns
the http proxy first. However we should prefer to use the https proxy
for general sockets, as it's more likely to support the CONNECT method.

Change-Id: I55dcadf2e142367e857f94e55fdbb0c4ddb513a3
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@nokia.com>
Reviewed-by: Robin Burchell <robin+qt@viroteck.net>
Reviewed-by: Richard J. Moore <rich@kde.org>
Reviewed-by: Shane Kearns <shane.kearns@accenture.com>