Features
   * Add server certificate selection callback near end of Client Hello.
     Register callback with mbedtls_ssl_conf_cert_cb().
   * Provide mechanism to reset handshake cert list by calling
     mbedtls_ssl_set_hs_own_cert() with NULL value for own_cert param.
   * Add accessor mbedtls_ssl_get_hs_sni() to retrieve SNI from within
     cert callback (mbedtls_ssl_conf_cert_cb()) during handshake.