Fix unportable coding in DetermineSleepTime().

We should not assume that struct timeval.tv_sec is a long, because
it ain't necessarily.  (POSIX says that it's a time_t, which might
well be 64 bits now or in the future; or for that matter might be
32 bits on machines with 64-bit longs.)  Per buildfarm member panther.

Back-patch to 9.3 where the dubious coding was introduced.

src/backend/postmaster/postmaster.c

@@ -1462,10 +1462,12 @@ DetermineSleepTime(struct timeval * timeout)
 
 	if (next_wakeup != 0)
 	{
+		long		secs;
 		int			microsecs;
 
 		TimestampDifference(GetCurrentTimestamp(), next_wakeup,
-							&timeout->tv_sec, &microsecs);
+							&secs, &microsecs);
+		timeout->tv_sec = secs;
 		timeout->tv_usec = microsecs;
 
 		/* Ensure we don't exceed one minute */