Fix Windows path bugs, run all config tests against Windows

This fixes a broken spec (see below) related to Ruby's Pathname class behaving differently on Windows vs. Unix when it comes to removing duplicate leading path separators in paths. The test was changed to not exercise that case (but still exercise non-leading duplicate path separators). @jkeiser, if we should cover the leading duplicate path separators case, let us know.

This adds to https://github.com/opscode/chef/pull/1942 and should replace it.

```
bundle exec rspec spec/unit/util/path_helper_spec.rb
...
Failures:

  1) Chef::Util::PathHelper on unix cleanpath removes extra slashes alone
     Failure/Error: PathHelper.cleanpath('//a///b/c/d/').should == '/a/b/c/d'
       expected: "/a/b/c/d"
            got: "//a/b/c/d" (using ==)
     # ./spec/unit/util/path_helper_spec.rb:84:in `block (4 levels) in <top (required)>'
```