Signed Kext Requirement

Creates a new requirement that dictates packages are unable to install due to requiring a signed kext to function.

Discussed this with @mikemcquaid earlier, so hopefully this is along the lines of what he had in mind and Mike doesn't think I've lost the plot too much :wink:.

Edit - *Much of what was in this original message stopped applying after the first couple of commits. The commit message has a better explanation that stays updated with each commit.*