Add nohttpsforwards option to captive portal

If the nohttpsforwards option is set, attempts to connect to SSL/HTTPS (Port 443) sites will not be forwarded to the captive portal.  This prevents certificate errors from being presented to the user even if HTTPS logins are enabled.  Users must attempt a connecton to an HTTP (Port 80) site to get forwarded to the captive portal. If HTTPS logins are enabled, the user will be redirected to the HTTPS login page.