url-vars.el (url-gateway-method): Add new method `tls'.

url-vars.el (url-gateway-method): Add new method `tls'.
url-news.el (url-snews): Use nntp-open-tls-stream if
url-gateway-method is tls.
url-ldap.el (url-ldap-certificate-formatter): Use
tls-certificate-information if ssl.el is not available.
url-https.el (url-https-create-secure-wrapper): Use tls if ssl
is not available.
url-gw.el (url-open-stream): Support tls url-gateway-method.
(url-open-stream): Likewise.