4.1.0nb1: Interix has no sync(2). Use fsync(2) there instead.

4.1.0nb1: Interix has no sync(2).  Use fsync(2) there instead.

While there, use fsync(2) on all hosts.  Without this, the fstime benchmark
will be disproportionately biased toward OS's and hardware that buffer
larger amounts of data in memory before [background] syncing.