Move root symlink check to engine.New

Since commit c91c365, when starting the docker daemon without an
existing /var/lib/docker directory, it fails with:

```
2013/12/18 23:39:36 Unable to canonicalize root (%!s(*string=0xc210077c80)): lstat /var/lib/docker: no such file or directory
```

Move the symlink checking code to engine.New after the root dir has been
created.