[Issue esp8266#3145] ESP8266mDNS: Improve compliance with DNS-SD RFC6763

- When the "_services._dns_sd._udp.local" meta-query is received, an
enumeration of services types is now returned (e.g. "_http._tcp.local")
instead of an enumeration of instances (e.g.
"MyService._http._tcp.local").  This is consistent with Section 9 of the
RFC.

- When a response is sent, the response records are now properly
partitioned as either answers or additional records.  Only response
records that were explicitly requested as a result of the questions
should be treated as answers.  This is consistent with Section 12 of the
RFC.

- The "MDNSResponder::advertiseServices()" method has been removed as it
was declared as "private" and is no longer being called.  This method
was sending a response on multiple interfaces when available, but this
wasn't really necessary since the interface from which the request was
received that caused it to be invoked is known.