remove dnsbl.zones, so overrides are better

[msimerson]

We need to think about the optimal default configuration for this plugin.

As it stands, if someone configures the plugin like this:

echo 'zones=b.barracudacentral.org, truncate.gbudb.net' >> config/dnsbl.ini

They'll also have zen.spamhaus.org configured as well, because it will be enabled in the invisible (to the user) config directory. This seems a POLA violation.

This PR isn't much better, in that if someone were to do this:

echo 'b.barracudacentral.org' > config/dnsbl.zones

They'd once again be surprised to find that zen.spamhaus.org is active.

suggestion #1

Remove the docs for dnsbl.zones and encourage everyone to use dnsbl.ini.

suggestion #2

Now that we can have empty values in our .ini files, we could also deprecate both methods and replace them with an INI section:

[zones]
b.barracudacentral.org
truncate.gbudb.net
zen.spamhaus.org

[smfreegard]

#1 gets my vote. Otherwise we break compatibility and I'd rather think about changing the config file when we introduce new functionality that might require it. e.g. you might want to look-up zen.spamhaus.org but only reject on 127.0.0.4 or .10 and .11 and raise some other flag for other hits etc. etc.