BUG/MEDIUM: dns: Release answer items when a DNS resolution is freed

BUG/MEDIUM: dns: Release answer items when a DNS resolution is freed

When a DNS resolution is freed, the remaining items in .ar_list and .answer_list
are also released. It must be done to avoid a memory leak. And it is the last
chance to release these objects. I've honestly no idea if there is a better
place to release them earlier. But at least, there is no more leak.

This patch should solve the issue #222. It must be backported, at least, as far
as 2.0, and probably, with caution, as far as 1.8 or 1.7.