SSL: support for per-certificate chains.

SSL: support for per-certificate chains.

The SSL_CTX_add0_chain_cert() function as introduced in OpenSSL 1.0.2 now
used instead of SSL_CTX_add_extra_chain_cert().

SSL_CTX_add_extra_chain_cert() adds extra certs for all certificates
in the context, while SSL_CTX_add0_chain_cert() only to a particular
certificate.  There is no difference unless multiple certificates are used,
though it is important when using multiple certificates.

Additionally, SSL_CTX_select_current_cert() is now called before using
a chain to make sure correct chain will be returned.