Only allow explicitly living players to trigger sector specials

Should resolve #236 (closed). I could not trigger the BCZ3 capsule while dead with this change.

changed the description

Wonder what playerstate was if it wasn't PST_DEAD and it wasn't PST_LIVE?

PST_REBORN?

Yes, I suspect that the player gets PST_REBORN to flag them as needing to respawn, and that in between the assignment and the respawn the sector special is processed.

merged

mentioned in commit 1caf92f0