I found that none of the daily, hourly, weekly or monthly snapshots were
being taken. When I looked into it I found that the scripts didn't
execute properly, because of the newly introduced check (9c6f065).
I'm not versed enough with POSIX to understand if there was some clever
intention with using exec than just calling which by it self, but it
works without exec so I removed it.
Split the cron file so that the hourly, daily, weekly, and monthly
zfs automatic snapshots still happen even if the system is offline
temporarily during the scheduled event, such as during a reboot.
On Debian systems, if anacron is installed, then it is used to run
the /etc/cron.{hourly,daily,weekly,monthly} directories, but not
the /etc/cron.d directory. This means that /etc/cron.d jobs are not
run if the system is offline when crond would usually invoke them.
