Fix gmtime and localtime on negative timestamps with fractional seconds (#3634)

The whole seconds were truncated toward zero while the fractional part
was measured from the floor, so any negative non-integral timestamp was
rendered one second late; gmtime was not monotonic across the epoch.

tests/optional.test

@@ -10,3 +10,11 @@ strftime("%A, %B %e, %Y")
 1435677542.822351
 "Tuesday, June 30, 2015"
 
+# Timestamps before the epoch are not supported on mingw/WIN32
+gmtime
+-1.25
+[1969,11,31,23,59,58.75,3,364]
+
+map(gmtime) | . == sort
+[-1.5,-1,-0.5,0,0.5,1,1.5]
+true