OSX/Freebsd/etc are incorrectly reported as Linux

Fixes an issue where any platform that was not Windows was considered Linux. Here is an example from my Macbook:

```
chef > require 'chef/sugar'
 => true 
chef > `uname`
 => "Darwin\n" 
chef > Chef::Sugar::PlatformFamily.linux?(node)
 => true 
```

I also add a sugar method called `nix?` which behaves in the exact manner that `linux?` used to behave.