Access denied 403 setting (appears to be) lost when forcing authentication on restricted pages

When the Force authentication setting is changed to On restricted pages the site_403 and securesite_403 variables are only set on the next anonymous page request (in _securesite_forced(), when empty($user->uid), in securesite_boot()). Before this happens, the Default 403 (access denied) page field on the Error reporting page will be cleared out (because securesite_403 isn't set yet).

Proposed solution is to set the respective variables immediately when the Secure Site settings form is submitted.