Flicky mapload crash fix (and other tmthing-related fixes)
This should fix #81 (closed), which was caused by the use of P_TeleportMove in A_FlickyCenter which was called by MF_RUNSPAWNFUNC during level load... which results in tmthing being set and never unset before the main gamelogic starts.
As a bonus related fix, I also safeguarded the following functions exposed to Lua so that they can't cause tmthing is set! errors either (since all of them directly or indirectly call P_CheckPosition or something that calls it):
P_ZMovementP_RingZMovementP_SceneryZMovementP_PlayerZMovementP_MovePlayer