Support automatic updates via Sparkle

[jakepetroules]

OpenConnect releases are distributed via GitHub Releases, it seems. OpenConnect should use Sparkle on macOS to automatically update itself.

The appcast file required by Sparkle can be hosted on the OpenConnect GitHub pages site, and should be updated each time a new release is made on GitHub Releases.

[horar]

In addition the 'Homebrew' cask system is supported for updates... Just now, is my focus to get more stable release packages on macOS (and some clean-up the code).

I will look later to this and come back ;-)