Fixing screen saver cancellation by icon hooks

 * The icon hooks ScreenSaverStart hook was not able to cancel
   the screen saver with a return code different than zero
   after at least one app was started
 * Kdesk was dragging a global IGNORE signal mode after executing an
   app, which caused the hook script execution to always return -1
 * Enabling the default signal mode (the system reports the return code)
   when execution the screen saver hook script