chpwd.js: Only update DOM properties if they're actually going to change. This might have a significant effect on the background-friendliness of the validation machinery. Certainly, if I open Firefox's developer tools, I used to see the various `whinge' elements highlighted as changing all the time, which was rather distracting if nothing else, and probably meant that DOM change-handling machinery was being engaged in order to do nothing of any use. Introduce a new function `update' which changes an object property only if its value would actually change, and use this in the `check' function.