switch-to-configuration-ng: Better handling of socket-activated units

Previously, if any unit had a socket associated with it, stc-ng
counted it as "socket-activated", meaning that the unit would get
stopped and the socket get restarted. That can wreak havoc on units
like systemd-udevd and systemd-networkd.

Instead, let units set the new flag notSocketActivated, which sets a
boolean on the unit indicating to stc-ng that the unit wants to be
treated like any other non-socket-activated unit instead. That will
stop/start or restart these units on upgrades, without unnecessarily
tearing down any machinery that the system needs to run.

nixos/lib/systemd-lib.nix

@@ -579,6 +579,8 @@ in rec {
       '' else "")
        + optionalString (def ? stopIfChanged && !def.stopIfChanged) ''
          X-StopIfChanged=false
+       '' + optionalString (def ? notSocketActivated && def.notSocketActivated) ''
+         X-NotSocketActivated=true
       '' + attrsToSection def.serviceConfig);
     };