Auto-detect default value for --forward-policy option in installers
    
    Forward policy defaults to 'first' if no IP address belonging to a private
    or reserved ranges is detected on local interfaces (RFC 6303).
    Defaults to only if a private IP address is detected.
    
    This prevents problems with BIND automatic empty zones because
    conflicting zones cannot be disabled unless forwarding policy == only.
    
    https://fedorahosted.org/freeipa/ticket/5710
    
    Reviewed-By: Martin Basti <mbasti@redhat.com>