These changes were modelled after the Owl version of vixie-cron,
but developed independently.
Our crontab used to send cron SIGUSR1 to tell cron to reread the
spool dir.  Now that crontab is not setuid root this doesn't work.
Instead, crontab pokes cron via a Unix domain socket located in the
tabs dir.
Please note, after these changes, the owner on user crontab files
will have to be changed manually from root to the uid of the
corresponding user for crontab to be usable.  cron itself will accept
tab files owned by either root or the user.
Also, any /var/cron/{allow,deny} files must be readable by group crontab.