Add a Lua global to tell if you are in the preticker

The preticker always runs for two tics at the start of a level. During this time leveltime does not increase, so there will be three tics at the start of a level where leveltime is zero. By checking defrosting you will be able to tell if the hook is running during the preticker, and rule out the extra two tics if you want.

defrosting reports the number of tics left for the preticker to finish. I.e. defrosting == 2 on the first frame of the preticker. defrosting == 0 if the preticker is not running.