OHAI-420: Add support for determining which package is running as init

Ports https://github.com/opscode/ohai/pull/108 to Ohai 7.

The only question is what happens on non-linux OSes. I believe the right thing is not to report `init_package` and fail as follows when queried for that attribute (might be prettier but for later version IMO): 

```
/Users/serdar/oc/oss/ohai/lib/ohai/system.rb:180:in `attributes_print': I cannot find an attribute named init_package! (ArgumentError)
	from /Users/serdar/oc/oss/ohai/lib/ohai/application.rb:94:in `block in run_application'
	from /Users/serdar/oc/oss/ohai/lib/ohai/application.rb:93:in `each'
	from /Users/serdar/oc/oss/ohai/lib/ohai/application.rb:93:in `run_application'
	from /Users/serdar/oc/oss/ohai/lib/ohai/application.rb:70:in `run'
	from /Users/serdar/oc/oss/ohai/bin/ohai:51:in `<top (required)>'
	from /Users/serdar/.rbenv/versions/2.1.1/lib/ruby/gems/2.1.0/bin/ohai:23:in `load'
	from /Users/serdar/.rbenv/versions/2.1.1/lib/ruby/gems/2.1.0/bin/ohai:23:in `<main>'
```

@danielsdeleo thoughts?