New virtual package: cron-daemon

New virtual package: cron-daemon

Create a virtual cron daemon package that:
 - Has to provide /usr/bin/crontab and support crontab entries
 - Correct execution of /etc/cron.d
 - Correct support of /etc/crontab
 - Support of crontab entries with names for days and months,
   ranges, step values
 - Correct execution of /etc/cron.{hourly,daily,weekly,monthly}

Signed-off-by: Javier Fernández-Sanguino Peña <jfs@computer.org>
Signed-off-by: Manoj Srivastava <srivasta@debian.org>