====== Reinstall original configuration file from package ======

1. Find out what package installed the config file:

<file>
$ dpkg -S unity-greeter.conf
  unity-greeter: /etc/lightdm/unity-greeter.conf
</file>

As you can see, the name of the package is unity-greeter.

2. Rename (or delete) the config file you wish to restore:

<file>
sudo mv -i /etc/lightdm/unity-greeter.conf /etc/lightdm/unity-greeter.conf.bak
</file>
 
3. Run the following command, replacing <package-name> with the name of the package:

<file>
sudo apt-get -o Dpkg::Options::="--force-confmiss" install --reinstall <package-name>
</file>

4. If everything worked as expected, you should get this message:

<file>
Configuration file `/etc/lightdm/unity-greeter.conf', does not exist on system. Installing new config file as you requested.
</file>

A Practical example when needing to reinstall all of the PulseAudio configuration files:

<file>
apt-cache pkgnames pulse |xargs -n 1 apt-get -o Dpkg::Options::="--force-confmiss" install --reinstall
</file>