xinetd: exec xinetd on launch

I noticed xinetd process doesn't get exec'd on launch, exec here so the bash
process doesn't stick around.

Signed-off-by: William Casarin <jb55@jb55.com>

nixos/modules/services/networking/xinetd.nix

@@ -146,7 +146,7 @@ in
       after = [ "network.target" ];
       wantedBy = [ "multi-user.target" ];
       path = [ pkgs.xinetd ];
-      script = "xinetd -syslog daemon -dontfork -stayalive -f ${configFile}";
+      script = "exec xinetd -syslog daemon -dontfork -stayalive -f ${configFile}";
     };
   };
 }