nixos/ids: Add service user ids

This should make it easier to rebase / merge commits for these services since the ids file is being updated constantly.
They are for multi-network support. Each tinc user is needed to run a different daemon for each network (similar to multiple openvpn servers). We could limit the number of users for now. The nixos service I'm working on dynamically determines how many users are available and limits the number of networks accordingly.