Can mDNS be opted out?

Is there anyway to disable the use of mDNS? My virtual machine doesn't support it.

````
$ ./webrtc-cli --offer --debug
Creating WebRTC peer...
Error: mDNS: failed to join multicast group
````

Start by reproducing the failure with `./webrtc-cli --offer --debug` in an environment where mDNS is unavailable. Trace where peer creation joins the mDNS multicast group and identify how a command-line opt-out could be exposed; done means the offer flow works without that join when disabled, while normal behavior remains unchanged.

Written by the indexing model from the issue text.