The systemd files included with syncthing are "library" and not
"configuration" units, so they should be placed in lib/systemd
rather than etc/systemd, so that they can be customised by creating
unit files in etc/. This also corresponds to how syncthing's own
build script installs them.
Currently only for the user services as NixOS handles the named system
instances slightly differently.
syncthing and syncthing-inotify are done the same way.
There are 4 parts to this:
1) Copy in the upstream unit files
2) Make the nixos module use the definition from upstream
3) Enable restarting of all instances (system and user) on resume
4) Allow the traffic in the firewall on default ports if wanted
fixes#18973
