user_cookie_save() has a weird/awkward function signature

Having read #440876: Reuse comment.module's anonymous cookie information, user_cookie_save()'s function signature only makes sense if it is passed form values. In that specific case, you rarely want to save all the form values to a cookie, so it makes sense to pass a second array listing just those keys you want to save.

The problem is, user_cookie_save() is not only being used to save form values to a cookie, and in my opinion the more popular use case for the function in contrib will not be to save form values to a cookie. It is in that case that the function signature is redundant and unwieldy.

Dries touched on this in #748976: "Show/hide descriptions" links no longer work and, when writing the patch for that issue I had a bit of a "*sigh* Drupal" moment when parsing user_cookie_save()'s signature.

This patch changes user_cookie_save()'s signature and updates all calls to it in core.

If we don't link the array_intersect_key pattern, we can have a function user_cookie_save_form_values (or some better name) that wraps user_cookie save and does the array_intersect_key in one place.

Admittedly,