Use corrected stat method for FreeBSD

This PR addresses the following when running specs on an actual FreeBSD machine:

```
  1) File "/data" should be owned by "my_account"
     On host `xxx.xxx.xxx'
     Failure/Error: it { should be_owned_by 'my_account' }
       expected `File "/data".owned_by?("my_account")` to return true, got false
       /bin/sh -c stat\ -c\ \%U\ /data\ \|\ grep\ --\ \\\^my_account\\\$

     # ./spec/xxx.xxx.xxx/files_spec.rb:15:in `block (3 levels) in <top (required)>'

  2) File "/data" should be grouped into "my_account"
     On host `xxx.xxx.xxx'
     Failure/Error: it { should be_grouped_into 'my_account' }
       expected `File "/data".grouped_into?("my_account")` to return true, got false
       /bin/sh -c stat\ -c\ \%G\ /data\ \|\ grep\ --\ \\\^my_account\\\$

     # ./spec/xxx.xxx.xxx/files_spec.rb:16:in `block (3 levels) in <top (required)>'
```

Somewhat strangely, the following new specs pass:
`bundle exe rspec spec/command/freebsd/file_spec.rb`

But, when run as part of the full suite the same specs fail:
`bundle exec rake spec:command`

I'm not sure why this is, and I thought I'd send the PR as a request for help. My initial investigation indicates the property[:os] is not updated by the `set :os, :family => 'freebsd'` line at the top of the file, but I'm not sure why. 

Thanks!