Improve VirtualBox shared folders + support vboxdrv service

The virtualbox shared folders code couldn't be skipped during runtime,
so add support boot option "novboxsf" to skip shared folders setup.

While at it reduce code complexity in config_virtualbox_shared_folders()
(by returning when not running under virtualbox).

Also do not hard code "grml" user to be added to vboxsf group, but rely
on $fstabuser instead.

If VirtualBox (not the guest additions, but the application itself) can
be started on Grml live system, then the /usr/bin/VBox exectuable is
present. The vboxdrv driver might not be loaded yet, take care of it via
vboxdrv.service. Also add user to vboxusers group. To be able to easily
skip it (via boot option "novbox") integrate in grml-autoconfig and not
into systemd presets.

This work was funded by Grml-Forensic.