Add support for DNSSEC validation
    
    Include ldns support for DNSSEC validation. It may then allow
    'VerifyHostKeyDNS yes' to skip manual validation of fingerprint, if
    machine has DNSSSEC support.
    
    ldns-config includes also python, which should not be part of ssh.
    Workaround by forcing /usr path, works similar as
    pkg-config --libs ldns.
    
    Allows command without asking:
    ssh -o "VerifyHostKeyDNS yes" pkgs.fedoraproject.org
    
    Signed-off-by: Petr Menšík <pemensik@redhat.com>