local: factor PreAllocate and SetSparse to lib/file

lib/file/preallocate_other.go

@@ -0,0 +1,15 @@
+//+build !windows,!linux
+
+package file
+
+import "os"
+
+// PreAllocate the file for performance reasons
+func PreAllocate(size int64, out *os.File) error {
+	return nil
+}
+
+// SetSparse makes the file be a sparse file
+func SetSparse(out *os.File) error {
+	return nil
+}