MFC: r261146
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.

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