ESP8266mDNS: Improve compliance with DNS-SD RFC6763

[ccooper21]

There are a few aspects of the existing ESP8266 MDNS implementation that are not compliant with RFC6763 describing the DNS-based Service Discovery (i.e. DNS-SD) protocol. Specifically:

• Section 9 states that when a service type enumeration request is received (i.e. a request with the special _services._dns-sd._udp.local meta-query), the response should be a set of PTR records identifying supported service types (e.g. _http._tcp.local). Presently, the MDNS implementation responds with a set of PTR records identifying service instances (e.g. MyService._http._tcp.local).

• Section 12 states that response records that were not explicitly requested by the client should be treated as additional records in the DNS response. This is reinforced by Section 4.1 of RFC1035 describing the DNS protocol. Presently, the MDNS implementation treats all response records as answers to the questions sent by a client, irrespective of the questions actually sent.