docker save: fix filemode permissions

currently the files created are not readable. This makes the files and
directories permissions more sane.

```
vbatts@jellyroll ~ (master *) $ docker save b | tar tv                             
drwx------ 0/0               0 2014-04-08 12:34 ./                                 
d--------- 0/0               0 2014-04-08 12:34 c44ffa1befab47e7242c5982512cf843d9040a639a2aaa9464acdaa7683055f0/
---------- 0/0               3 2014-04-08 12:34 c44ffa1befab47e7242c5982512cf843d9040a639a2aaa9464acdaa7683055f0/VERSION
---------- 0/0             638 2014-04-08 12:34 c44ffa1befab47e7242c5982512cf843d9040a639a2aaa9464acdaa7683055f0/json
-rw-r--r-- 0/0         2684416 2014-04-08 12:34 c44ffa1befab47e7242c5982512cf843d9040a639a2aaa9464acdaa7683055f0/layer.tar
---------- 0/0              83 2014-04-08 12:34 repositories                       
vbatts@jellyroll ~ (master *) $ /home/vbatts/src/docker/docker/bundles/0.9.1-dev/dynbinary/docker-0.9.1-dev save b | tar tv
drwx------ 0/0               0 2014-04-08 12:38 ./                                 
drwxr-xr-x 0/0               0 2014-04-08 12:38 c44ffa1befab47e7242c5982512cf843d9040a639a2aaa9464acdaa7683055f0/
-rw-r--r-- 0/0               3 2014-04-08 12:38 c44ffa1befab47e7242c5982512cf843d9040a639a2aaa9464acdaa7683055f0/VERSION
-rw-r--r-- 0/0             638 2014-04-08 12:38 c44ffa1befab47e7242c5982512cf843d9040a639a2aaa9464acdaa7683055f0/json
-rw-r--r-- 0/0         2684416 2014-04-08 12:38 c44ffa1befab47e7242c5982512cf843d9040a639a2aaa9464acdaa7683055f0/layer.tar
-rw-r--r-- 0/0              83 2014-04-08 12:38 repositories                       
```

Docker-DCO-1.1-Signed-off-by: Vincent Batts <vbatts@redhat.com> (github: vbatts)