use buffered writing for InRelease splitting

Hardly noticeable, but given that we have the option to easily enable
it, lets enable it as every newline in the message is written
individually by the code.

apt-pkg/contrib/gpgv.cc

@@ -341,7 +341,7 @@ bool OpenMaybeClearSignedFile(std::string const &ClearSignedFileName, FileFd &Me
    unlink(message);
    free(message);
 
-   MessageFile.OpenDescriptor(messageFd, FileFd::ReadWrite, true);
+   MessageFile.OpenDescriptor(messageFd, FileFd::ReadWrite | FileFd::BufferedWrite, true);
    if (MessageFile.Failed() == true)
       return _error->Error("Couldn't open temporary file to work with %s", ClearSignedFileName.c_str());