Optimized NoCollide, cleaned up StackerCalcPos, added more cvars

The nocollide functionality should now complete in  !(N-1) ops as
compared to N^2.

Got rid of unnecessary parameters within the StackerCalcPos function.

Added stacker_force_freeze/weld/nocollide so that server
operators/admins can force stacked props to be spawned
frozen/welded/nocollided.