mod_dnsupdate: Rewrite port config vs DNS comparison code I'm not sure if it was correct, which means it was hard to understand and thus needed to be simplified. Hope this accomplishes that.

This module blocks configured queries that change server state.

E.g. to make vCard storage read-only:

``` lua
readonly_stores = {
	vcard = { "vcard-temp", "vCard" };
}
```