overlays: freed from the shackles of flakes - #585

setting 'accept-flake-config' is ignored: iets never applies a flake's nixConfig, so a flake that sets extra-substituters or allow-import-from-derivation there runs without them; put the settings in nix.conf
setting 'plugin-files' is ignored: iets loads no plugins
Firewall port configurations are set but will not be applied.
system-manager: the following units declare `aliases`, but system-manager does not support systemd unit aliases: the option is accepted yet nothing is emitted for it. Use a second unit definition if you need the alias name to exist. See https://github.com/… Affected units: ssh-system-manager.service