Out-of-date Z-Flag in DNS headers (AD, CD are missing)

Currently, the Z flag in the DNS struct spans 3 bits. In the most recent IANA definition, however, the Z flag only uses 1 bit, the last three being used for DNSSEC-related purposes (I think):

| Bit | Flag | Description | Reference |
|-------|------|--------------------------|----------------------------------------|
| bit 5 | AA | Authoritative Answer | [RFC1035] |
| bit 6 | TC | Truncated Response | [RFC1035] |
| bit 7 | RD | Recursion Desired | [RFC1035] |
| bit 8 | RA | Recursion Available | [RFC1035] |
| bit 9 | | Reserved | |
| bit 10| AD | Authentic Data | [RFC4035][RFC6840][RFC Errata 4924] |
| bit 11| CD | Checking Disabled | [RFC4035][RFC6840][RFC Errata 4927] |

https://www.iana.org/assignments/dns-parameters/dns-parameters.xhtml#dns-parameters-6
https://www.freesoft.org/CIE/RFC/2065/40.htm

Even though this means a breaking change, I believe is semantically important. Beyond the fact that we're not up-to-date with IANA spec, the implication of the Z flag is that it's assumed to be 'zero', which currently doesn't hold anymore.

I'd be happy to implement this change, but I'm curious to see what people think about it!