Use `AI_CANONNAME' if it's defined.

Use `AI_CANONNAME' if it's defined.

This will follow PTR records. `www.verplant.org' is changed to `verplant.org',
for example. It's not what I intended (I wanted a reverse lookup of
IP-addresses), but still better than what we had originally. I don't know if
I'll leave it like this. Might be though..