fileutil: Sync on HFS/OSX needs to be handled differently.

A call file.Sync on OSX doesn't guarantee actual persistence on
physical drive media as the data can be cached in physical drive's
buffers. Hence calls to file.Sync need to be replaced with
fcntl(F_FULLFSYNC).

pkg/fileutil/sync_linux.go

@@ -21,6 +21,11 @@ import (
 	"syscall"
 )
 
+// Fsync is a wrapper around file.Sync(). Special handling is needed on darwin platform.
+func Fsync(f *os.File) error {
+	return f.Sync()
+}
+
 // Fdatasync is similar to fsync(), but does not flush modified metadata
 // unless that metadata is needed in order to allow a subsequent data retrieval
 // to be correctly handled.