httpGetAuthString() was added with cups 1.3 and we depend on a newer version
now. The direct field access was a fallback in case httpGetAuthString()
was missing, so this can also be dropped.
Ported to master from !938
The http* family of functions was deprecated after CUPS 1.7. We can
conditionally use it when built against a newer version of CUPS. The
additional parameters are taken directly from the fallback values
inside CUPS itself.
