stime.2, time.2, utimensat.2, ctime.3, difftime.3, ftime.3, getspnam.3, mq_receive...

stime.2, time.2, utimensat.2, ctime.3, difftime.3, ftime.3, getspnam.3, mq_receive.3, mq_send.3, rtime.3, sem_wait.3, strftime.3, strptime.3, timeradd.3, rtc.4, core.5, proc.5, icmp.7, time.7: Global fix: Consistently define the Epoch

All definitions of the Epoch have been refactored to the following:

  1970-01-01 00:00:00 +0000 (UTC)

That form is more consistent, logical, precise, and internationally
recognizable than the other variants.

Also, some wording has been altered as well.

Signed-off-by: Michael Witten <mfwitten@gmail.com>
Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>