Add DNS Support

Add DNS Support instead of only supporting Namecoin.

I suggest to use TXT records similar to the following example:

```
subdomain IN TXT "zero=1Gfey7wVXXg1rxk751TBTxLJwhddDNfcdp"
```

The issue names no files, tests, or entry points; begin by locating the existing Namecoin resolution path. Define and verify DNS TXT-record support using the example format, with the work considered done when DNS can replace Namecoin for the described lookup.

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