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

Also known as "status_request" or OCSP stapling. This is defined in RFC6066
section 8.

TODO:
 * make OCSP_basic_verify() work
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_resp_import() work
 * properly verify OCSP response
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.