openssl: Don't ignore CA paths when using Windows CA store

openssl: Don't ignore CA paths when using Windows CA store

This commit changes the behavior of CURLSSLOPT_NATIVE_CA so that it does
not override CURLOPT_CAINFO / CURLOPT_CAPATH, or the hardcoded default
locations. Instead the CA store can now be used at the same time.

The change is due to the impending release. The issue is still being
discussed. The behavior of CURLSSLOPT_NATIVE_CA is subject to change and
is now documented as experimental.

Ref: bc052cc (parent commit)
Ref: https://github.com/curl/curl/issues/5585