cron: Implement -s command option

[otis]

Summary

Based on an idea suggested by Job Snijders <[e-mail]>,
a -s command option has been implemented, with great deal of inspiration
from OpenBSD.

When used, it allows only a single instance of given command
to run at once, allowing the next one to run only after the previous
one finishes.

For this to happen, job queue has been reworked to use queue instead
of a simple linked list.

[otis]

rebase to recent main

[gbe]

The man page part looks good, but I am a little bit confused, on my FreeBSD systems I see

root 66156 0.0 0.0 12800 1680 - Is 7Jul21 0:04.38 /usr/sbin/cron -s

when executing 'ps axuw'. So it seems, that some kind of a '-s' option is already implemented.

[pstef]

In D31146#701876, @gbe wrote:

The man page part looks good, but I am a little bit confused, on my FreeBSD systems I see

root 66156 0.0 0.0 12800 1680 - Is 7Jul21 0:04.38 /usr/sbin/cron -s

when executing 'ps axuw'. So it seems, that some kind of a '-s' option is already implemented.

That's a command option for cron, the change has to do with crontab entries, see crontab.5 and its example:

# run every minute, only send mail if ping fails
* * * * *       -n ping -c 1 freebsd.org

usr.sbin/cron/cron/job.c

26

This is in the wrong order, if I'm not misunderstanding style(9).

36

Shouldn't old code be removed instead of commented out?

[otis]

Fix style(9)

[bcr]

Approving the man page part.
Hope we can get it committed soon.