Rework handling of stuck entities in engine movetypes

Rework handling of stuck entities in engine movetypes

Avoids most false positives and unnecessary work.
Mentions likely code path and cvar deprecations.

Fixes nades ending up where they shouldn't, which looked like:
Unstuck entity 450 (classname "ogre_grenade") with offset 0.000000 0.000000 28.000000.

Allows certain entities to move while stuck in the world (see comments).
Fixes https://github.com/DarkPlacesEngine/darkplaces/issues/18

Signed-off-by: bones_was_here <bones_was_here@xonotic.au>