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
MFC after:	1 weeks

sys/x86/isa/atrtc.c

@@ -354,7 +354,7 @@ atrtc_gettime(device_t dev, struct timespec *ts)
 #ifdef USE_RTC_CENTURY
 	ct.year += readrtc(RTC_CENTURY) * 100;
 #else
-	ct.year += 2000;
+	ct.year += (ct.year < 80 ? 2000 : 1900);
 #endif
 	critical_exit();
 	/* Set dow = -1 because some clocks don't set it correctly. */