adds defaultText for all options that use `cfg.*` values in their
defaults, but only for interpolations with no extra processing (other
than toString where necessary)
Since https://github.com/NixOS/nixpkgs/pull/61321, local-fs.target is
part of sysinit.target again, meaning units without
DefaultDependencies=no will automatically depend on it, and the manual
set dependencies can be dropped.