Retrieves the current user object.
Will set the current user, if the current user is not set. The current user will be set to the logged-in person. If no user is logged-in, then it will set the current user to 0, which is invalid and won't have any permissions.
This function is used by the pluggable functions wp_get_current_user() and get_currentuserinfo(), the latter of which is deprecated but used for backward compatibility.
Current WP_User instance.