systemd-services: add saned

Bonus points for figuring out what --with-systemd does in sane's automake...

• https://gitlab.archlinux.org/archlinux/packaging/packages/sane/-/tree/main?ref_type=heads
• Upstream service files https://gitlab.com/sane-project/backends/-/tree/master/frontend?ref_type=heads

changed milestone to %initial systemd support

added difficulty::easy systemd labels

assigned to @jane400

@jane400 this is the scanner thing, right? I see you added this to the 'initial systemd support' milestone but I'm not sure if this is strictly required for that... My initial feeling is that it's not required (so should remove from milestone), but I'm open to being proven wrong :D

removed milestone %initial systemd support