Allow multiple listeners (e.g. for IPv4 & IPv6)

[cbix]

As of currently (bc3bb04), shaman doesn't allow listening on multiple addresses. This is especially bad if I want it to run over both IPv4 and IPv6 on the same host. Now I'd have to keep separate configurations and API endpoints for separate instances of shaman.

Not sure there is a simple solution using the existing github.com/miekg/dns listener, but if you spawn multiple of these in goroutines and read their return values from a channel asynchronously, that would probably be a 5 LOC solution on that part. Ofc it still needs to be integrated in the configuration and CLI flags.

[danhunsaker]

None of the Nanobox stack, Nanopack included, actually supports IPv6 properly. There's little indication it ever will, now that App Sail is out and using DO's internal infrastructure instead of something provider agnostic like shaman.

[cbix]

Just saying, Shaman works for me over IPv6 and I could run it listening on an IPv6 address, just not in parallel to listening on an IPv4 address inside the same instance.