MFC r304570,r321235:

r304570 (by trasz):

Add the "-n" flag to cron(8), to prevent it from daemonizing.
This makes it possible to use it with external supervisors.
The "-n" flag name is compatible with Linux, NetBSD, and OpenBSD.

r321235:

Fix trivial whitespace bug introduced in usage message changes for -n
support (r304570).

MFC: r261146

Bad timespec_subtract() calculations produce negative tv_nsec on i386
which cause EINVAL returned from nanosleep() which cause loop in
cron_sleep() and making all cron jobs to start about 30 seconds earlier
(which cause f.e. logfiles rotation by newsyslog delayed by 1 hour).

Use simple and proved calculations from kernel's timespecsub() instead.

MFC: r261146

Bad timespec_subtract() calculations produce negative tv_nsec on i386
which cause EINVAL returned from nanosleep() which cause loop in
cron_sleep() and making all cron jobs to start about 30 seconds earlier
(which cause f.e. logfiles rotation by newsyslog delayed by 1 hour).

Use simple and proved calculations from kernel's timespecsub() instead.