Fix listmodes when the config does not specify a wildcard size entry.

Currently, the wildcard entry is only appended if the user does not specify any `<banlist>` tags. This causes the limit to be effectively zero. This is in direct conflict with the documentation in the example configuration file which states:

> If none are specified or no maxbans tag is matched, the banlist size defaults to 64 entries.

This fixes #896 reported by @md-5.
Should add a constructor to ListLimit while we're at it.