The current upstream virtualbox-guest-additions ebuild is a bit weird – in fact, simply running the virtualbox-guest-additions auto-installer on the CD you can mount within virtualbox does a better job in general.

Reasons to rewrite:

1. bundled installer of guest-additions uses an /opt directory and Gentoo's does not, so it is just set up very differently.
2. gentoo's ebuild has various issues (documented in other sub-tasks)
3. gentoo's ebuild should ideally be compatible with the virtualbox default paths.