SSL: avoid SSL_CTX_set_tmp_rsa_callback() call with LibreSSL. LibreSSL removed support for export ciphers and a call to SSL_CTX_set_tmp_rsa_callback() results in an error left in the error queue. This caused alerts "ignoring stale global SSL error (...called a function you should not call) while SSL handshaking" on a first connection in each worker process.