Work on properly implementing the activation of systemd services.

README.md

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+
+## Profile generation
+
+
+## Activation strategy
+The activation script calls `system-manager activate`,
+which will perform the following actions.
+
+### Systemd services
+The info about services (name and store path of the service file) is found
+in a file called `services/services.json` in the system-manager configuration directory.
+The info about the services that were part of the previous generation is stored
+in a state file at `/var/lib/system-manager`.
+We then:
+1. Compare the list of services present in the current configuration with the
+   ones stored in the state file from the previous generation.
+1. For all services in the new generation,
+   create a symlink from `/etc/systemd/system/<service name>` to the service file
+   in the nix store.
+1. For all services present in the old generation but not in the new one:
+   1. Stop the service.
+   1. Remove the symlink from `/etc/systemd/system`.
+1. Perform a systemd daemon-reload
+1. Start the services that are present in this generation and not in the previous one
+1. Restart services that are present in both
+
+This approach basically ignores the `wantedBy` option.
+A future version might improve upon this, but one of the complexities is that
+NixOS does not encode the `wantedBy` option in the generated unit files, but
+rather produces `<unit name>.wants` directories in the directory that
+`/etc/systemd/system` gets linked to.
+Supporting this properly would mean that we need to find a way to register
+the `wantedBy` option on a non-NixOS system in a way such that we can use it.
+
+### Udev rules
+
+
+### Files under `/etc`