ACME config cleanups (#4525)

* Handle listening for ACME requests on IPv6 addresses

the weird url-but-not-actually-a-url-string doesn't handle IPv6 addresses
without extra quoting. Building a string which you are about to parse again
seems like a weird choice. Let's just use listenTCP, which is consistent with
what we do elsewhere.

* Clean up the default ACME config

make it look a bit more consistent with everything else, and tweak the defaults
to listen on port 80.

* newsfile