lib/strtoday.c: strtoday(): Fix calculation

lib/strtoday.c: strtoday(): Fix calculation

Days officially roll over at 00:00 UTC, not at 12:00 UTC.  I see no
reason to add that half day.

Also, remove the comment.  It's likely to get stale.

So, get_date() gets the number of seconds since the Epoch.  I wonder how
that thing works, but I'll assume it's something similar to getdate(3)
+ mktime(3).  After that, we need to convert seconds since Epoch to days
since Epoch.  That should be a simple division, AFAICS, since Epoch is
"1970‐01‐01 00:00:00 +0000 (UTC)".  See mktime(3).

Fixes: 45c6603cc86c ("[svn-upgrade] Integrating new upstream version, shadow (19990709)")
Link: <https://github.com/shadow-maint/shadow/issues/939>
Reported-by: Michael Vetter <jubalh@iodoru.org>
Tested-by: Gus Kenion <https://github.com/kenion>
Signed-off-by: Alejandro Colomar <alx@kernel.org>