posix timers: Check for overflow when converting to ns

[markj]

Summary

Disallow a time or timer period value when the conversion to nanoseconds
would overflow. Otherwise it is possible to trigger a divison by zero
in realtime_expire_l(), where we compute the number of overruns by
dividing by the interval.

Fixes: 7995dae9 ("posix timers: Improve the overrun calculation")
Reported by: [e-mail]
Reported by: [e-mail]
MFC after: 1 week

[kib]

I think we want some symbol like ONE_SEC_NS, so that reader can be freed from counting zeroes in the constant.

[markj]

Add NSEC_PER_SEC, use it throughout kern_time.c.

[kib]

sys/kern/kern_time.c

75

The 'L' suffix is not needed, I believe.