Add debconf / dbconfig support to debian8 folder

Features added during package installation: 
- Possibility to install a local or a remote database
- The user can set a custom password for the mysql application account (zmuser), if empty an auto generated password will be used
- The user can set a custom password for the ZoneMinder default administrative account (admin), if empty the default password will be used (admin)
- The user can choose the webserver to configure (currently only apache is supported but the list could be extended)

Feature added during package removal:
- Possibility to remove the database based on user decision

Feature added during package purge:
- Possibility to delete remaining files in /var/cache/zoneminder based on user decision (confirmation required)

During package upgrade the database is updated by dbconfig.
I kept the call to zmupdate.pl at the end of the postinst script.
This is redundant but at least this shows that database upgrade is successfull.

The package can be reconfigured with `sudo dpkg-reconfigure zoneminder`.
The user can change the database location when reconfiguring.

I added the french translation for all debconf prompts.

Based on debian policy for web applications, I also change the file permissions of /etc/zm/zm.conf.