Makefile: Install configuration files.

Configuration files are installed only if there are no files at the
destination already.  This means that you get at least a skeleton
installation from the source distribution, even if it requires some
light hacking.

This also requires:

  * moving some files from the Debian packaging into the main
    distribution;

  * allowing the user to configure a `userv' installation directory; and

  * adjusting the Debian installation runes.