(UDMF prereq.) Spawn Z refactor

Refactors the object spawn z coordinate calculation code into a separate function, save for the rings and hoops. The code now only makes use of mapthing_t.options in P_PrepareRawThings() in order to obtain mapthing_t.z, which will be used from then onwards instead.

This change might break Lua scripts which make use of the mapthing.z, though it shouldn't be a problem given it was inconsistent prior to this merge.

Also, ring-like objects such as emerald tokens or emblem no longer ignore the MF_SPAWNCEILING flag. Not that any of them have it anyway.