dns: introduce lookupService function

Uses getnameinfo to resolve an address an port into a hostname and
service.

NOTE: latest libuv needs to be pulled into node before merging this.

Does what it says on the tin. I'm not thrilled about the `lookupService` name, suggestions are welcome :-)

Suggested reviewer: @indutny, but all feedback is of course welcome!
If it should be a run-time (i.e. always) sanity check rather than a debug check, then yes, CHECK() is what you want.