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.