FOF sector special fixes

Originally, this was just to fix FOFs with the Bouncy FOF sector special still being able to bounce players off them even when made to disappear (i.e not have FF_EXISTS; one way of doing this is using linedef type 445 on them). However, I also spotted that Space Countdown probably has the same problem and so I fixed this too. Sector specials should not work on FOFs that do not "exist"!

EDIT: Updated to include fixes for the Each Time trigger specials as well as object-carrying scrollers.