Set mode flag to 0 when not using O_CREATE in Unix.openfile

The mode was 0644 but in OCaml that's decimal and not octal, the
correct integer literal would have been 0o644. However, as the calls
don't use O_CREAT, it doesn't matter. Setting the parameter to 0 is
cleaner.

unix/tar_lwt_unix.ml

@@ -125,7 +125,7 @@ module Archive = struct
       | None -> Lwt.return_unit
       | Some hdr ->
         let filename = dest hdr.Tar.Header.file_name in
-        Lwt_unix.openfile filename [Unix.O_WRONLY] 0644 >>= fun ofd ->
+        Lwt_unix.openfile filename [Unix.O_WRONLY] 0 >>= fun ofd ->
         copy_n ifd ofd hdr.Tar.Header.file_size >>= fun () ->
         Reader.skip ifd (Tar.Header.compute_zero_padding_length hdr) >>= fun () ->
         loop () in
@@ -143,7 +143,7 @@ module Archive = struct
         header_of_file filename >>= fun hdr ->
 
         write_block hdr (fun ofd ->
-            Lwt_unix.openfile filename [Unix.O_RDONLY] 0644 >>= fun ifd ->
+            Lwt_unix.openfile filename [Unix.O_RDONLY] 0 >>= fun ifd ->
             copy_n ifd ofd hdr.Tar.Header.file_size
           ) ofd
       end in

unix/tar_unix.ml

@@ -83,7 +83,7 @@ module Archive = struct
   let extract dest ifd =
     let dest hdr =
       let filename = dest hdr.Tar.Header.file_name in
-      Unix.openfile filename [Unix.O_WRONLY] 0644
+      Unix.openfile filename [Unix.O_WRONLY] 0
     in
     extract_gen dest ifd
 
@@ -100,7 +100,7 @@ module Archive = struct
         else
           let hdr = header_of_file filename in
           Some (hdr, (fun ofd ->
-              let ifd = Unix.openfile filename [Unix.O_RDONLY] 0644 in
+              let ifd = Unix.openfile filename [Unix.O_RDONLY] 0 in
               copy_n ifd ofd hdr.Tar.Header.file_size))
       in
       List.filter_map f files