1. Find out what package installed the config file:
$ dpkg -S unity-greeter.conf unity-greeter: /etc/lightdm/unity-greeter.conf
As you can see, the name of the package is unity-greeter.
2. Rename (or delete) the config file you wish to restore:
sudo mv -i /etc/lightdm/unity-greeter.conf /etc/lightdm/unity-greeter.conf.bak
3. Run the following command, replacing <package-name> with the name of the package:
sudo apt-get -o Dpkg::Options::="--force-confmiss" install --reinstall <package-name>
4. If everything worked as expected, you should get this message:
Configuration file `/etc/lightdm/unity-greeter.conf', does not exist on system. Installing new config file as you requested.
A Practical example when needing to reinstall all of the PulseAudio configuration files:
apt-cache pkgnames pulse |xargs -n 1 apt-get -o Dpkg::Options::="--force-confmiss" install --reinstall