Bug#767894: More permission issues

[Alexandre Detiste]

The current status of /var/spool/cron/crontabs is undetermined ...
users either inherit what was setup up by the previosu cron daemon or
get a vanilla 0755 folder
on fresh install.

e.g.: http://anonscm.debian.org/cgit/pkg-cron/pkg-cron.git/tree/debian/postinst

Having crontab translate on the fly user crontabs into ~/.config/systemd/user
service & timers (systemd --user) would avoid all these security problems.

This need a refactor of the generator to turn it into a module that
can be called from crontab.

crontab -l & -e would just parse back the "Description=[cron] " records