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Execute commands periodically

CROND command.
/etc/crond

crond is a daemon that executes commands at preset times.  The
commands and their scheduled execution times are kept in the 
/usr/spool/cron/crontabs/file_name, where file_name is the user name
from /etc/passwd. To preserve compatibility with cron 3.2.0
crond looks for /usr/lib/crontab first. If file exist crond runs 
as cron 3.2.0 and never looks for the user's tables. Otherwise,
crond looks for user's tables and never will look for /usr/lib/crontab.

When crond execute user crontab it changed effective id to user id,
and cd to user $HOME directory.

crond should be executed only once (at boot time). /usr/lib/cron/FIFO
is used as a lock file to prevent the execution more than one crond.

If mail options is enabled (default) crond will send mail to the
owner of crontab about all commands that failed. 

To allow crond to remove lock file /usr/lib/cron do not send signal
KILL to crond. You can use signal TERM. 
crond ignores signals INT, HUP, & PIPE. crond uses ALRM signal by itself. 


	Files and directorys.

/usr/lib/cron			main cron directory.
	700 root root
/usr/lib/cron/FIFO		lock file (named pipe).
	created by crond; removed by crond/rc
/usr/lib/cron/cron.allow	list of allowed users.
	600 root root
/usr/lib/cron/cron.deny		list of denied users.
	600 root root
/usr/lib/crontab		old crontab entry. 
/usr/spool/cron			spool directory parent
	700 root root
/usr/spool/cron/crontabs	spool directory
	700 root root

#/usr/lib/cron/log		accounting information (not used).
#/etc/default/cron		user config. file (not used).

NOTICE: These two lines should be added to the /etc/rc file:
		rm -f /usr/lib/cron/FIFO
		/etc/crond&
The line:
		/etc/cron&
should be removed.

Bugs: crond does not write log file. Crond will do it as soon as I will know
how this log file should look, and what the reason is to have it.
The size of the hostname + domain should not exceed 1000 characters.

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