SSL: made it possible to iterate though all certificates. A pointer to a previously configured certificate now stored in a certificate. This makes it possible to iterate though all certificates configured in the SSL context. This is now used to configure OCSP stapling for all certificates, and in ngx_ssl_session_id_context(). As SSL_CTX_use_certificate() frees previously loaded certificate of the same type, and we have no way to find out if it's the case, X509_free() calls are now posponed till ngx_ssl_cleanup_ctx(). Note that in OpenSSL 1.0.2+ this can be done without storing things in exdata using the SSL_CTX_set_current_cert() and SSL_CTX_get0_certificate() functions. These are not yet available in all supported versions though, so it's easier to continue to use exdata for now.

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