Remove MT_NULL hacks

Fixes #721 (closed)

Since hooklib refactor, mobjtype of mobj passed to mobj hook is compared with NULL as part of a general check to determine if the hook being called is actually a mobj hook. MT_NULL mobjs break this of course and end up calling a non mobj hook instead.

I don't think MT_NULL should ever be spawned so I removed the instances of that in the codebase. I added MT_RAY as a general purpose mobj to replace P_TryMove hacks and created P_CheckMove to nicely package the hack. MT_NULL can no longer be spawned, may God rest its soul...

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