STOP THE CLOCK linedef exectutor!

* When activated, stops the timer in SP/MP.
* Applies to the mapheader countdowntimer as well.
* If you're playing Record Attack, also exits the level immediately.
* It has no special modes, no linedef flags, no parameters, nothing.
* Only not an innate property of A_BossDeath because people may want it to NOT happen sometimes, or make it happen with non-boss events too.

Also, skip over calling P_DoPlayerExit if the player isn't in game.