Add forward policy "none".
    
    This policy adds ability to disable forwarding on per-zone basics.
    Now it is possible to forward all queries to global forwarders
    and selectively disable forwarding for specific master zones.
    
    It will be handy in situation where global forwarder is not able
    to resolve records from some sub-zones served by internal DNS
    servers.
    
    Logging and error handling related to forwarders was cleaned up.
    
    https://fedorahosted.org/bind-dyndb-ldap/ticket/98
    
    Signed-off-by: Petr Spacek <pspacek@redhat.com>