community/virtualbox-guest-additions: upgrade to 6.1.18 & fix auto-resize guest display

I didn't get how to add VBoxClient to .init, because of errors on trying demonize it.

Another approach, run it from /etc/xdg/autostart/xxx.display not works, because daemon started as x session user, do not working too.

I noticed that this feature works in Arch and adopted their approach with VBoxDRMClient here.

I'm totaly newbie, and I guess someone can review my work.

If MR will be merged, I'm planning to edit wiki docs with instructions how to start virtualbox-drm-client service.