snapper: various tweaks and fixes

Fix numerous configuration files referring to ‘/usr’ and ‘/lib’.

Some paths were still ending up in ‘/nix/store/.../nix/store/...’,
despite some well-intended hacks meant to avoid that. Replace them
with other hacks. It's all very fragile and ugly, so snapper should
feel right at home.

Oh, and `snapper create-config ~` still won't actually *do*
anything, because D-Bus (#12452). Use `--no-dbus` and add files
to ‘/etc’ as long as it complains.

Only fair that I help maintain this mess.