chpwd.js: Only update DOM properties if they're actually going to change.

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.