MFC: r284931

Set the initial system time to a sane (as in: not end of 21st century)
value when booting on a PC with CMOS clock set to a year before 2000.

This uses 1980 (instead of 1970 as in the initial patch) as pivot year as
suggested by imp in the PR followup.

PR: 195703
Submitted by: cs@soi.spb.ru
Reviewed by: imp
Approved by: re (gjb)