net: mdns_responder: Set socket of service for statistics purposes

If the dispatcher is muxing the connection i.e., so there are two
services for the same port, then mark service socket descriptor
of the service with the socket number so that "net sockets" shell
command can show a proper value for it.

Signed-off-by: Jukka Rissanen <jukka.rissanen@nordicsemi.no>