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>
I used selectors because version and release are also selectors - so just to stay with one type instead of mixing all them. But its no problem to change ALL of them later. If you need them, create another issue and i'll happily do that change!