[sftp] Add additional sftpProtocol::openConnection() for libssh 0.8.3+

Summary:
This uses the new known_hosts API and creates SHA256 fingerprints for
public key verifcation.

Signed-off-by: Andreas Schneider <asn@cryptomilk.org>

Differential Revision: https://phabricator.kde.org/D16538

(cherry picked from commit f857c73b9a23a32f6dcb8d4199fc1820482b3df1)