Bug 766606 - [mozilla.org] Firefox download button doesn't block new unsupported operating systems

* Copied the old platform detection code from download.js
* Added Windows 2000 (NT 5.0 and 5.0.1) detection
* Added Windows XP SP1 detection
Merge pull request #975 from kyoshino/bug-766606-unsupported-os

Bug 766606 - [mozilla.org] Firefox download button doesn't block new unsupported operating systems