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: 45c6603cc8 ("[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>

lib/strtoday.c

@@ -68,10 +68,9 @@ long strtoday (const char *str)
 		return retdate;
 	}
 
-	t = get_date (str, NULL);
+	t = get_date(str, NULL);
 	if ((time_t) - 1 == t) {
 		return -2;
 	}
-	/* convert seconds to days since 1970-01-01 */
-	return (t + DAY / 2) / DAY;
+	return t / DAY;
 }