Add Linux support

This adds a basic Linux provider, as requested in [MODULES-1560](https://tickets.puppetlabs.com/browse/MODULES-1560).  It does not presently update the spec tests to handle that.  Suggestions on which tests to look at fixing are welcome.
It's probably fine to be left here for parity with the linux provider.

When we are done here, you could add the Linux support into the ACL module ;)