This adds a mdns_service to mtransport to handle responding to mDNS queries.
All hostnames will be generated from UUIDs, so the responder assumes that it
is the only responder for a hostname which is registered with it. Because of
this, the responder does not first make a DNS query itself to see if any other
responder is handling a hostname, and does not wait a random amount of time
before replying, both of which are required by the specification to avoid
collisions with other responders.
Differential Revision: https://phabricator.services.mozilla.com/D38489
d668f1db2e