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.
(cherry picked from commit e37383db1f55ad21f200903c18f9750753e05f00)
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.
(cherry picked from commit 1431eecd20c020b9c0ad826993015bc8957e82d6)