Allow SFTP support to create empty remote directories without uploading a file

FTP supports creating remote directories by just changing into them, with SFTP we need an upload in order kick into the create directory state
Copy the 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.
Merge branch 'darwinssl-get-trust'