IPv6 tweaks

Upgraders will have `ip` fields in `txp_discuss` and
`txp_discuss_ipban` revised to `VARCHAR(45)`.

On the grounds that new installs will process `txpsql.php` and then
chew through the patches in `textpatten/update` (see
http://forum.textpattern.com/viewtopic.php?pid=284631#p284631 for
reference), `ip` in `txp_log` is taken care of by `_to_4.5.7.php` file.
This update will take care of the other two places `ip` exists.

This should resolve
https://github.com/textpattern/textpattern/issues/442, which can be
closed if this gets the thumbs up.