implement dns resolution of hostnames at startup

resolves hostnames to multiple ip addresses prior to calling
server_new().
update resolution to be explicit, and only on any_of clusters
Merge branch 'cm-resolve-hostnames' of https://github.com/WrathOfChris/carbon-c-relay
usedns: misc fixes/changes

- renamed to useall
- avoid duplicate getaddrinfo code blocks
- avoid double resolves by reusing the first resolve
- show resolved/expanded/useall-ed addresses
- show IPv6 addresses correctly