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.
Yes.  It's also available in the various BSD flavors including Darwin/OS X with the same syntax and in the same location.  AIX, HP-UX, and Solaris seem to have somewhat differing syntax, but I believe they have the same command location.