ndns

Now it throw an exception when the lable.size() is larger than one, indicating that we should use
addMultiLevelLabelRrset instead on those labels.

However, it is actually wrong in that, some labels, should just have size larger than 1, without AUTH records.

For example, for a d-key:
/parentZone/NDNS/childzone/dkey-1/CERT/version
It has multiLevelLabel: childzone/dkey-1
But we don't want a childzone AUTH record, which will override the real NS record.