Fix and speedup timestamp calculations which is roughly based on the patch in the mentioned PR:
Fix and speedup timestamp calculations which is roughly based on the patch in
the mentioned PR:

- bounds check time->month as it is used as an array index
- fix usage of time->month as array index (month is 1-12)
- fix calculation based on time->day (day is 1-31)
- fix the speedup code as it doesn't calculate correct timestamps before
  the year 2000 and reduce the number of calculation in the year-by-year code
- speedup month calculations by replacing the array content with cumulative
  values
- add microseconds calculation
- fix an endian problem

PR:		kern/97786
Submitted by:	Andriy Gapon <avg@topspin.kiev.ua>
Reviewed by:	scottl (earlier version)
Approved by:	emax (mentor)
MFC after:	1 week

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