apt::force: Added 2 parameters for automatic configuration file handling...

...; fixes #modules-1306

when updating or installing newer packages with apt::force and you have changed previous
configuration files aptitude or apt-get will prompt what to do. You can suppress that
by pre-define the action with cfg_files parameter (new, old or unchanged and its backward
compatible if not defined). With a second optional parameter cfg_missing you can force
your provider to install missing configuration files as well.

Signed-off-by: Martin Seener <martin@seener.de>
Just for consistency, can you quote all of the valid values here?