vmnet: support mDNS

[AkihiroSuda]

Adding mDNS will be useful for host<->guest and guest<->guest communication.

[jandubois]

I was already planning to look into something like https://github.com/blake/external-mdns for use with kubernetes, so it would be great to find something that will work for all use cases. I can't remember if running multiple mDNS services on a single interface was a problem or not. 😄

[AkihiroSuda]

I can't remember if running multiple mDNS services on a single interface was a problem or not. 😄

Seems safe

https://superuser.com/questions/1272967/is-it-possible-to-run-multiple-mdns-responders-for-dns-sd-on-the-same-host
https://stackoverflow.com/a/14388707

The meaning of SO_REUSEADDR changes for multicast addresses as it allows multiple sockets to be bound to exactly the same combination of source multicast address and port. In other words, for multicast addresses SO_REUSEADDR behaves exactly as SO_REUSEPORT for unicast addresses. Actually, the code treats SO_REUSEADDR and SO_REUSEPORT identically for multicast addresses, that means you could say that SO_REUSEADDR implies SO_REUSEPORT for all multicast addresses and the other way round.