SSH configuration fixes.
    
    Use GlobalKnownHostsFile instead of GlobalKnownHostsFile2 in ssh_config, as the
    latter has been deprecated in OpenSSH 5.9.
    
    If DNS host key verification is enabled, restrict the set of allowed host
    public key algorithms to ssh-rsa and ssh-dss, as DNS SSHFP records support only
    these algorithms.
    
    Make sure public key user authentication is enabled in both ssh and sshd.
    
    ticket 2769