SSL: enabled TLSv1.3 with BoringSSL. BoringSSL currently requires SSL_CTX_set_max_proto_version(TLS1_3_VERSION) to be able to enable TLS 1.3. This is because by default max protocol version is set to TLS 1.2, and the SSL_OP_NO_* options are merely used as a blacklist within the version range specified using the SSL_CTX_set_min_proto_version() and SSL_CTX_set_max_proto_version() functions. With this change, we now call SSL_CTX_set_max_proto_version() with an explicit maximum version set. This enables TLS 1.3 with BoringSSL. As a side effect, this change also limits maximum protocol version to the newest protocol we know about, TLS 1.3. This seems to be a good change, as enabling unknown protocols might have unexpected results. Additionally, we now explicitly call SSL_CTX_set_min_proto_version() with 0. This is expected to help with Debian system-wide default of MinProtocol set to TLSv1.2, see http://mailman.nginx.org/pipermail/nginx-ru/2017-October/060411.html. Note that there is no SSL_CTX_set_min_proto_version macro in BoringSSL, so we call SSL_CTX_set_min_proto_version() and SSL_CTX_set_max_proto_version() as long as the TLS1_3_VERSION macro is defined.