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. There is (currently) no reference to `ip` in the `_to_4.6.0.php`, but if these changes belong in there instead, please advise and I'll raise a more appropriate PR.

This should resolve https://github.com/textpattern/textpattern/issues/442, which can be closed if this gets the thumbs up.