* make i2d_X509 and d2i_X509 always available
i2d_X509 and d2i_X509 have been available in all versions of OpenSSL, so
make them available even if nimDisableCertificateValidation is set.
* introduce getPeerCertificates, fixes#13299
getPeerCertificates retrieves the verified certificate chain of the peer
we are connected to through an SSL-wrapped Socket/AsyncSocket. This
introduces the new type Certificate which stores a DER-encoded X509 certificate.