API changes
    * Modify semantics of `mbedtls_ssl_conf_[opaque_]psk()`:
      In Mbed TLS 2.X, the API prescribes that later calls overwrite
      the effect of earlier calls. In Mbed TLS 3.0, calling
      `mbedtls_ssl_conf_[opaque_]psk()` more than once will fail,
      leaving the PSK that was configured first intact.
      Support for more than one PSK may be added in 3.X.