Introduce DNS-SD using mDNS

[scaprile]

mg_mdns_listen() signature is changed to the standard mg_*_listen(&mgr, fn, fn_data)
mDNS name can then be specified as fn_data, and fn be NULL.
If fn is not NULL, it is called for every mDNS record request. If fn_data is also not NULL, it is checked before callingfn. So:

• Passing only fn_data works as before, with the addition of an extra argument, only mDNS, Mongoose replies to a given name
• Passing fn and fn_data, the user event handler can respond to requests associated to its name, including services, so supporting DNS-SD the easiest way, as Mongoose handles all parsing and response generation; the user only has to match service names to his database.
• Passing only fn, the user event handler is called for every mDNS request, this allows a more flexible way to do whatever weird handling might be needed, as Mongoose does not check for names