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-unit-options.nix

@@ -535,6 +535,18 @@ in rec {
         '';
       };
 
+      notSocketActivated = mkOption {
+        type = types.bool;
+        default = false;
+        description = ''
+          If set, a changed unit is never assumed to be
+          socket-activated on configuration activation, even if
+          it might have associated socket units. Instead, the unit
+          will be restarted (or stopped/started) as if it had no
+          associated sockets.
+        '';
+      };
+
       startAt = mkOption {
         type = with types; either str (listOf str);
         default = [];