Restore feature parity to linux and mac (now with better diffs!)

- Remove things from linux that were removed from mac in 8ad09c3
- Keep fancy_echo lines the same on linux and mac, both for a consistent
  user experience, and for easier diffs
- Move platform-specific sections to top
- Update mac fancy_echo to match linux fancy_echo (fancy colors!)
- In linux-prerequisites, use exit code 1 if not supported

![Screenshot of diff between `linux` and `mac`](https://f.cloud.github.com/assets/552093/376148/44bd5164-a406-11e2-87f9-5c5e7ab112d5.png)