Add support for NSEC3.
    
    NSEC3PARAM is loaded from NSEC3PARAMRecord attribute at zone apex
    in LDAP. Default (when the attribute is not present) is to use NSEC.
    
    Limitation: Only one NSEC3PARAM record per zone is supported at the moment.
    
    https://fedorahosted.org/bind-dyndb-ldap/ticket/56
    
    Signed-off-by: Petr Spacek <pspacek@redhat.com>