Copy the darwinssl SSLContext peer trust into the PureInfo struct

When `CURLE_SSL_CACERT` is returned callers can extract `CURLINFO_SSL_TRUST` to get the `SecTrustRef` suitable for presenting to the user and modifying the keychain trust settings.

I saw Nick Ziztmann discuss adding a new info selector in <http://curl.haxx.se/mail/lib-2012-06/0334.html> but don’t think this code was ever published.