Make URL pass the TLS peer status to the caller

* lisp/url/url-http.el (url-http-parse-headers): Pass the GnuTLS
status of the connection to the caller.
Make the https logic in url-http work on redirects

(url-http-parse-headers): When being redirected, make sure we
flush the previous certificate.
Make eww mark valid/invalid https pages

* lisp/net/eww.el (eww-update-header-line-format): Mark valid/invalid
certificates in the header line.
(eww-invalid-certificate, eww-valid-certificate): New faces.