WIP: gtls: add support for the Certificate Status Request TLS extension

Also known as "status_request" or OCSP stapling. This is defined in RFC6066
section 8, and requires GnuTLS 3.1.3 or higher.

TODO:
 * make GnuTLS OCSP support actually work.
nss: add support for the Certificate Status Request TLS extension

Also known as "status_request" or OCSP stapling. This is defined in RFC6066
section 8, and requires NSS 3.15 or higher.
curl: add --cert-status option

This enable the CURLOPT_SSL_VERIFYSTATUS functionality.