int _gsasl_plain_client_step(Gsasl_session *sctx, void *mech_data _GL_UNUSED, const char *input _GL_UNUSED, size_t input_len _GL_UNUSED, char **output, size_t *output_len)
int _gsasl_plain_server_step(Gsasl_session *sctx, void *mech_data, const char *input, size_t input_len, char **output, size_t *output_len)