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!
Right, I figured it was something like that.  The problem with that is that the tests may run inside a network where a DNS query always resolves to _something_.  Then again, this is probably not the only test that's susceptible to such issues.