VBox guest additions. Refs #282
Update VBox Additions code with some minor changes (especially for cache-ability)

Also, this includes a hacky init script that bfirsh gets to take all the credit for that tries to mount `/Users` and/or `/c/Users`, since we can't run `VBoxService` proper (thanks to hairy issues with 32bit userspace of TCL; good times).
Merge pull request #534 from tianon/vbox-guest-additions

VirtualBox Guest Additions