Only jump back in a menu to the nearest previous block with subpictures if a seek operation is already in progress.

This should prevent infinite loops on some discs (e.g. Independence Day) where the menu highlight packets are somehow not processed before the navigation data indicates that buttons must be shown.
Prevent MythWelcome blocking input devices when calling mythshutdown to avoid resetting the idle timer (see [cac1413]) when trying to determine whether the backend is idle, thereby preventing shutdown.