Ensure the blacklist lookup uses a fully qualified domain name

Without the trailing '.' the lookup might have a local domain appended
per /etc/resolv.conf on a poxsix system, for example.  This can cause
spurious failures.

I ran into this in an environment where we have a wildcard MX record
in place.  The result was that dns_get_record() returned a result for
<host>.<blacklist>.<our-domain> and subsequently treated every URL as
blacklisted. ;)