libssh2_session: Add support for creating known_hosts file

libssh2_session: Add support for creating known_hosts file

The libssh2 code wasn't supposed to create the known_hosts file, but
recent findings show, that we can't use the default created by OpenSSH
as libssh2 might damage it. We need to create a private known_hosts file
in the config path.

This patch adds support for skipping error if the known_hosts file is
not present and let libssh2 create a new one.