Configurable time to preserve files in /var/crash

The value of 7 days is hardcoded into `/etc/cron.daily/apport`, but some users might prefer that these files be retained longer.

If modifying `/etc/cron.daily/apport` is an acceptable way of customising this, then it might suffice to have a shell variable to define the desired number of days, and then use that in the find commands (rather than hardcoding `+7` into the find commands), eg. as in the example attached patch.