PATCH: Don't blacklist packages that contain modified config files if dpkg is not going to prompt on them

Recently I discovered that rsync was not automatically upgraded with a security update on one of my boxes but it was updated on others. The box runs an rsync server and obviously it had a modified /etc/rsyncd.conf and running unattended-upgrade -d revealed that due to this fact it wasn't being updated. However, I also have configured APT to supply dpkg with such command-line options that no config file prompt would occur. It doesn't make sense to blacklist packages that contain modified config files if APT/dpkg is configured in this way. This functionality can be achieved by including the following snippet in the apt.conf:

Dpkg {
        Options { "--force-confold" };
};

The attached patch will modify unattended-update to do that. If you like the idea, feel free to use it as is or as an inspiration for better implementation of the idea.