csi: check for private marker in 'S' case

csi: check for private marker in 'S' case

The handler for 'S' final character does not check for a private
marker. This can cause a conflict with a sequence called 'XTSMGRAPHICS'
which also has an 'S' final character, but uses the private marker '?'.
Without checking for a private marker, st will perform a scroll up
operation when XTSMGRAPHICS is seen, which can cause unexpected display
artifacts.