Merge branch 'vboxguest-sbin-removal'.

This removes all references to .../sbin for the guest additions and also
installs all binaries to .../bin instead (so no more .../sbin).

The main motivation for doing this is commit 98cedb3d (which
unfortunately had to be reverted in a9f2e107) and pull request #9063,
where the latter is an initial effort to move mount.vboxsf to .../bin
instead of .../sbin.

The commit I made afterwards is finishing the removal of .../sbin
entirely.