OCSP stapling: loading OCSP responses.

OCSP stapling: loading OCSP responses.

This includes the ssl_stapling_responder directive (defaults to OCSP
responder set in certificate's AIA extension).

OCSP response for a given certificate is requested once we get at least
one connection with certificate_status extension in ClientHello, and
certificate status won't be sent in the connection in question.  This due
to limitations in the OpenSSL API (certificate status callback is blocking).

Note: SSL_CTX_use_certificate_chain_file() was reimplemented as it doesn't
allow to access the certificate loaded via SSL_CTX.