Mac networking: check system keychain for proxy auth

... and not when normal HTTP authentication is required. Also,
query the system keychain for the right credentials depending
on the URL scheme.

Task-number: QTBUG-30434
Change-Id: Ib6f74029b2e0de9734497440e3b0e48cdf73adcb
Reviewed-by: Richard J. Moore <rich@kde.org>
Mac networking: only try system proxy credentials once

... instead of running into an endless loop in case they are wrong.

Task-number: QTBUG-30434
Change-Id: Iab258ebe1098a0c95f19da789a7a86de9d5bf149
Reviewed-by: Richard J. Moore <rich@kde.org>