Un-deprecate get_current_user_id

[filosofo]

There is not a direct, reliable way to access the current user's ID. You either have to get the user object and then the ID property if present, or assume that the $current_user global is populated already (and that's not a reliable assumption).

get_current_user_id is the perfect solution and existed (imperfectly) in MU. Rather than deprecating it, we should move it to general WP and make it use the WP pluggable user functions.

[ryan]

(In [15174]) Update get_current_user_id() and move to user.php. Props filosofo. fixes #13785