Restore handling relative targets in the link resource

Fixes https://github.com/opscode/chef/issues/2580.

After staring at the resource & provider for a while, I think the best way to fix this is to revert https://github.com/opscode/chef/commit/b4cbc0835d4020082b1ecdc6d52ca19e5236da22. 

We need to make sure that we pass the relative path in `to` to the `symlink` functions as it is so that systems can setup a relative symbolic link which can be portable when the directory moves. 

Thoughts @randomcamel, @lamont-granquist, @jdmundrawala, @adamedx ?
Whether it's needed or not, putting the new parameter in the middle will break any existing callers that think they're passing the boolean for `add_prefix`.