Fix installation of vbox guest additions.

Credits for this idea go to @myhro who suggested emulating uname.

I cleaned his changes up somewhat and moved the script into a
separate file to make things easier to look at.

I did a test build of wheezy with my changes and the modules were
installed correctly.

root@localhost:/home/vagrant# dkms status
vboxguest, 4.3.20, 3.2.0-4-amd64, x86_64: installed

root@localhost:/home/vagrant# lsmod | egrep ^vbox
vboxsf                 33359  0
vboxvideo              12437  0
vboxguest             162115  1 vboxsf