dns: fix c-ares error reporting regression
The test case from the previous commit exposed a regression in the way
that c-ares errors are reported to JS land. Said regression was
introduced in commit 756b622 ("src: add multi-context support").
Fixes the following test failure:
$ out/Release/node test/simple/test-dns-regress-6244
util.js:675
var errname = uv.errname(err);
^
Error: err >= 0
at Object.exports._errnoException (util.js:675:20)
at errnoException (dns.js:43:15)
at Object.onresolve [as oncomplete] (dns.js:145:19)
lib/dns.js erroneously assumed that the error code was a libuv error
code when it's really a c-ares status code. Libuv handles getaddrinfo()
style lookups (which is by far the most common type of lookup), that's
why this bug wasn't discovered earlier.