sane: fix multifunctional devices rights conflict with cups

[abbradar]

Another part of #4857 -- this makes MFDs work both with cups and sane. Without this they were being assigned to 'scanner' group, which prevented CUPS from working with them at all. This is how it's fixed on Arch Linux IIRC. Tested.

[lucabrunox]

What's the other relevant udev rule that should have more priority? Just curiosity, seems fine otherwise.

[abbradar]

50-udev-default.rules. It sets lp as an owner group for all USB printers.

[lucabrunox]

And why there's no problem with having group lp instead of scanner?

[abbradar]

libsane works with access rights of an end user, while cupsd is a daemon which runs as cups/lp. This can't be fixed by adding cups to needed group because cupsd internally doesn't set additional groups when dropping privileges. I have a patch which fixes that, but that would be much more intrusive than just to change ownership like that.

[lucabrunox]

Ok thanks :)