systemd: make ctrl-alt-del target configurable. (#16911)

We currently only allow upstream's default of "reboot.target" due to the
way the symlinks are initialized. I made this configurable similar to the
default unit.

nixos/modules/system/boot/systemd-lib.nix

@@ -176,7 +176,7 @@ rec {
       ${optionalString (type == "system") ''
         # Stupid misc. symlinks.
         ln -s ${cfg.defaultUnit} $out/default.target
-
+        ln -s ${cfg.ctrlAltDelUnit} $out/ctrl-alt-del.target
         ln -s rescue.target $out/kbrequest.target
 
         mkdir -p $out/getty.target.wants/

nixos/modules/system/boot/systemd.nix

@@ -120,7 +120,6 @@ let
       "systemd-poweroff.service"
       "halt.target"
       "systemd-halt.service"
-      "ctrl-alt-del.target"
       "shutdown.target"
       "umount.target"
       "final.target"
@@ -162,7 +161,6 @@ let
       "systemd-hostnamed.service"
       "systemd-binfmt.service"
     ]
-
     ++ cfg.additionalUpstreamSystemUnits;
 
   upstreamSystemWants =
@@ -485,6 +483,15 @@ in
       description = "Default unit started when the system boots.";
     };
 
+    systemd.ctrlAltDelUnit = mkOption {
+      default = "reboot.target";
+      type = types.str;
+      example = "poweroff.target";
+      description = ''
+        Target that should be started when Ctrl-Alt-Delete is pressed.
+      '';
+    };
+
     systemd.globalEnvironment = mkOption {
       type = types.attrs;
       default = {};