Bugfix: use apt-get install -s to simulate package install

 * works successfully to probe if a package is at all available
 * h/t @fh
Bugfix: only write settings to an .ini

 * php5-easybib-apc and php5-easybib-apcu distribute an apc.ini themselves
 * this fixes the problem when we overwrite it and need to guess apcu.so or apc.so