Fix multiple issues with alternatives module
Fix multiple issues with alternatives module

 - Changes are no longer erroneously reported on RHEL (#12)
 - Adding new link groups on Debian works again.
   - This was broken in a previous commit by assuming the OS was RHEL
     if `update-alternatives --query <name>` had a return code of 2
 - Prefer `--display` over `--query` for determining available
   alternatives
   - --display is more distro-agnostic and simplifies the code
 - Fix missing `msg=` in `fail_json` call when `link` is missing
 - Document that `link` is required on RHEL-based distros

Tested on Ubuntu 12.04+ and CentOS 6/7

thaim/ansible,thaim/ansible