nixos/switch-to-configuration: reload user units

When rebuilding you have to manually run `systemctl --user
daemon-reload`. It gathers all authenticated users using
`loginctl list-user` and runs `daemon-reload` for each of them.

This is a first step towards a `nixos-rebuild` which is able to reload
user units from systemd. The entire task is fairly hard, however I
consider this patch usable as it allows to restart units without running
`daemon-reload` for each authenticated user.