Fix graph/tags_unit_test.go on FreeBSD

- Directories should have u+x (0600 -> 0700)
- Don't chown to root.root by default because it requires root privilege

Docker-DCO-1.1-Signed-off-by: Kato Kazuyoshi <kato.kazuyoshi@gmail.com> (github: kzys)
I was running the test as a normal user (not root) and probably that's why. Even on Linux, the test works for just root.

```
docker@boot2docker:~$ uname -a
Linux boot2docker 3.13.3-tinycore64 #1 SMP Wed Mar 12 05:35:38 UTC 2014 x86_64 GNU/Linux
docker@boot2docker:~$ whoami
docker
docker@boot2docker:~$ mkdir testdir
docker@boot2docker:~$ chmod 600 testdir
docker@boot2docker:~$ cd testdir
-sh: cd: can't cd to testdir
docker@boot2docker:~$ ls -ld testdir
drw-------    2 docker   staff           40 Jul  4 01:41 testdir/
docker@boot2docker:~$ sudo sh
root@boot2docker:/home/docker# cd testdir
root@boot2docker:/home/docker/testdir# pwd
/home/docker/testdir
root@boot2docker:/home/docker/testdir# 
```