Clean-up the DNSBL checking function a little bit and its call site.

Always log (a notice-level log message) when there's a DNSBL match, along with
the address it actually resolved to (indicates which list the IP address was
found in).
No log message otherwise.

Note: resolve_ip() doesn't actually support IPv6 yet, so I don't think this
IPv6 address parsing/reformatting logic has actually been tested.