Since there seem to be a few of these DNS-related tags let's collect them all in one place

* [tag:mx-record] <- <s>[tag:mx]</s>, 
* [tag:a-record]  <- <s>[tag:arecord]</s>
* [tag:soa-record] <- <s>(Selective retagging of stuff in [tag:soa])</s>
* [tag:cname-record] <- <s>[tag:cname]</s>
* [tag:ns-record] <- <s>[tag:ns]</s>